Two judgments have so far been handed down in the matter of R (Ammori) v Secretary of State for the Home Department; the challenge to the Home Secretary’s decision to proscribe Palestine Action as a terrorist organisation under the Terrorism Act 2000 (‘the 2000 Act’). The first was on the matter of interim relief, which was rejected ([2025] EWHC 1708 (Admin)). The second concerned leave to seek judicial review, which was granted ([2025] EWHC 2013 (Admin)). For ease of reference, I will refer to the two judgments as ‘IJ’ (for ‘Interim Judgment’) and ‘PJ’ (for ‘Permission Judgment’) respectively. The Home Secretary has since appealed the granting of permission, arguing on procedural grounds that judicial review should not be available because of the availability of appeal through the statutory scheme. The Court of Appeal’s judgment on that question is expected next month. In the meantime, however, there are lessons to be drawn from the two judgments handed down already, lessons which are relevant both to the ongoing Palestine Action litigation itself and more generally to public law adjudication in the national security context.

In this post, I briefly outline both the grounds on which Ms Ammori’s application has been allowed to proceed and the grounds that were rejected. I then focus on the High Court’s treatment of one of these rejected grounds: the claim that the Home Secretary acted ultra vires the powers granted to her in the 2000 Act, because Palestine Action’s activities did not fall under the definition of ‘terrorism’ in section 1 of that legislation. The Court gave short shrift to this argument, encouraging lawyers for Ms Ammori to instead reformulate the claim as an improper purpose one. The main claim of this post is that this move muddied the waters on this particular ground, and speaks to a broader failure on the part of the Court to take seriously its obligation to construe the Terrorism Act 2000 in line with the principle of legality.

The Litigation so Far

In the first judgment, the Court held that the harm that would ensue if interim relief were refused did not outweigh the public interest in maintaining the force of the proscription order. In the second judgment, the Court granted Ms Ammori permission to apply for judicial review on two of the eight grounds put forward. The first claim which raised a serious issue to be tried, according to Mr Justice Chamberlain, is that the proscription order was contrary to section 6 of the Human Rights Act 1998, because it is incompatible with Articles 10, 11 and 14 ECHR. The second claim was that the decision was taken in breach of natural justice because Palestine Action was not consulted in advance of the decision.

The claims in respect of which permission was not granted were as follows. First, that the decision was unlawful because it was either ultra vires or because it was taken subject to an improper purpose. Second, that the Home Secretary erred in law, because direct action groups like Palestine Action are not concerned with influencing government policy, as is required by the 2000 Act. Third, that the Home Secretary failed to gather sufficient information on Palestine Action’s activities, the impact of proscription on particular individuals, and its impact on other direct action and protest groups. Fourth, that the Home Secretary took into account irrelevant considerations, including the views of pro-Israel lobby groups. Fifth, that the Home Secretary failed, by neglecting to undertake an assessment of the proportionality of proscription, to apply her own policy. Sixth, that the Home Secretary was in breach of the public sector equality duty in section 149 of the Equality Act 2010.

There is much to be unpacked with each of the grounds above. In the remainder of this post, I want to focus on only one of the rejected grounds; namely, the claim that the order was ‘ultra vires and/or made for an improper purpose’ (IJ [63]). There was some confusion around this ground, and the relationship between ultra vires and improper purpose claims. More significantly, the Court, in my view, gave inadequate consideration to the claim that the principle of legality required the 2000 Act be interpreted consistently with common law principles.

The Development of the Ultra Vires/Improper Purpose Claim in Ammori

Section 3(4) of the 2000 Act provides that the Home Secretary may proscribe an organisation if she ‘believes that it is concerned in terrorism’. ‘Terrorism’ is given a layered definition in section 1 of the statute. It involves action that is ‘designed to influence the government [or an international governmental organisation] or to intimidate the public or a section of the public’, and is undertaken ‘for the purpose of advancing a political, religious, racial or ideological cause’.

The action must meet one of the following requirements, set out in s 1(2) in order to fall under the definition:

(a)        involves serious violence against a person,

(b)        involves serious damage to property,

(c)        endangers a person’s life, other than that of the person committing the action,

(d)        creates a serious risk to the health or safety of the public or a section of the public, or

(e)        is designed seriously to interfere with or seriously to disrupt an electronic system.

Subsection (b) is the relevant one here. The Home Secretary claimed that because Palestine Action took action involving serious damage to property, and because it did so in an effort to influence government policy and for the purpose of advancing a political cause, it could lawfully be proscribed as a terrorist organisation.

Lawyers for Ms Ammori argued that the legislation could not lawfully be used to proscribe a direct action group like Palestine Action. The way that this claim was articulated, however, changed slightly between the application for interim relief and the application for permission to seek judicial review. Here is the claim as articulated in the interim relief judgment:

Ground 1 is that the order is ultra vires and/or was made for an improper purpose. The ultra vires case is that the legislation does not on its true construction authorise proscription of a ‘direct action civil disobedience network’ such as PA. The improper purpose case is that the Secretary of State acted for an improper purpose by exercising the power conferred by s. 3(3) of the 2000 Act in relation to such a group. (IJ [62])

There are really two separate, though related, claims here. The first is a claim about the scope of section 1(2)(b) of the 2000 Act. By arguing that the proscription order was ultra vires, Ms Ammori was arguing that groups like Palestine Action simply did not fall within the scope of the legislation, properly construed. The second claim is a distinct one; that even if Palestine Action fell within the scope of section 1, the Home Secretary abused the discretionary power granted to her by the legislation by acting with an improper purpose.

In the judgment on interim leave, Mr Justice Chamberlain expressed scepticism about the ultra vires variation of this argument: ‘On its face, the language used is clear and the context does not make it any less so’ (IJ [64]). Likely in response to this prompting from the Court, in its skeleton argument at the leave application, the lawyers for Palestine Action effectively dropped the ultra vires variation of the arguments. Now, it was accepted that Palestine Action fell within the ‘four corners’ of the statute, but it was claimed that the Home Secretary nevertheless acted with an improper purpose in proscribing the organisation. This argument was rejected in the judgment granting leave (PJ [73]-[75]).

The treatment of this claim merits a closer look. The first version of the claim, submitted in the application for interim relief, is unclear on the relationship between ultra vires and improper purpose claims: ‘the order is ultra vires and/or was made for an improper purpose’. In the reformulated version of the argument, however, in which lawyers for Palestine Action seemed to drop the ultra vires part of the claim and focus instead on improper purpose, the argument still reads as speaking to ultra vires rather than improper purpose. They argued, for example, that the Home Secretary could not use the discretion given to her by the 2000 Act for the purposes of banning an organisation whose actions are for the most part lawful, which enjoys broad popular support, and whose actions are directed at preventing atrocities and crimes under international law. These do not really read as claims about the Home Secretary having acted with improper purpose. They do not, notably, speak to the reasons the Home Secretary might have had for proscribing Palestine Action. Other claims submitted under the heading of ‘irrelevant considerations’ – that the Home Secretary took into account the views of pro-Israel lobby groups, for example – speak to the purpose behind the proscription in a way that the claims submitted under the ‘improper purpose’ heading do not. In part, this illustrates the ways in which these grounds of review – illegality, improper purpose and relevant/irrelevant considerations – collapse into one another in practice. This somewhat muddled articulation of an ‘improper purpose’ claim, however, can be understood in the context of the clearly-signalled reluctance of the Court to give proper consideration to a claim of illegality.

In what follows, my main contention is that the Court erred in steering the lawyers for Palestine Action away from an ultra vires claim. The strongest version of this claim was not that the Home Secretary acted with an improper purpose (though conceivably this might have been argued as well), but rather that the Home Secretary acted ultra vires the powers granted in the 2000 Act. After making this argument, I will give some broader thoughts on the approach to statutory construction in the national security context. The courts have, I contest, been insufficiently clear on the operation of the principle of legality in this context. The proper interpretation of legislation like the Terrorism Act 2000, like any other piece of legislation, is constrained by common law principles. In the course of adopting a highly deferential posture towards the executive in the national security context, the courts have frequently been insufficiently attentive to their obligation to interpret statues in light of those fundamental principles.

Ultra Vires, the Principle of Legality, and the National Security Context

It is a well-known tenet of public law that statutory provisions must be read, where possible, consistently with common law rights and principles. The original submission on behalf of Ms Ammori, recall, was that the proscription order was ‘ultra vires and/or was made for an improper purpose’. The proscription of a protest organisation clearly impacts the common law right to freedom of speech, both for members of Palestine Action and the many people subsequently arrested under sections 12 and 13 of the 2000 Act for support of the organisation. The strongest version of the ultra vires claim, then, is that these common law rights and principles constrained the proper interpretation of section 1 of the Terrorism Act, such that the conduct of Palestine Action did not meet the definition in that provision.  Their subsequent proscription would then have been ultra vires the power granted to the Home Secretary in section 3 of the Act.

The main difficulty facing Ms Ammori’s argument was that section 1(2)(b) of the 2000 Act includes in its definition of terrorism any action that ‘involves serious damage to property’. She would need to argue that common law principles constrain the legal meaning of this provision, such that the kind of property damage Palestine Action engages in falls outside the ‘serious damage to property’ covered by the provision. For example, it could be argued that the ‘property damage’ in s 1(2)(b) applies only to property damage that endangers lives. If Palestine Action did not fall within the definition of terrorism under s 1(2), then the Home Secretary would be acting beyond the proscription powers given to her in section 3.

Mr Justice Chamberlain, however, gave short shrift to this notion:

… it is sometimes legitimate to interpret a statute more narrowly than its express language suggests, for example in accordance with the principle of legality or consistently with unincorporated international law. But neither the principle of legality, nor the principle that legislation should be interpreted in accordance with unincorporated international law allows a domestic court to interpret legislation contrary to its express language, read in context. In the present case, it is not possible to read that language as incorporating a restriction on the use of the power against ‘civil society or dissent groups’ if the groups concerned fall within the definition in the Act. If Parliament had intended such a restriction, it would have included it expressly. If it had done so, it would no doubt have defined “civil society or dissent group”, a phrase whose meaning is far from clear. It is not the function of the court to rewrite statutory powers by introducing vague and undefined limitations such as these. (IJ [65])

The judge is here rejecting the suggestion that some other constraint should be read into this provision in order to render the provision consistent with common law principles. According to Mr Justice Chamberlain, the ‘express language’ of the statute leaves no room for such an interpretation.

This conclusion, however, is far from obvious. It is true that in R v Gul, the Supreme Court was reluctant to read constraints on section 1, seemingly viewing the articulation of such constraints as a matter for Parliament. More recently, however, in Miranda v SSHD, the Court of Appeal read a mens rea requirement into s 1(2). Moreover, analogously explicit statutory language is found in other parts of the courts’ jurisprudence on the principle of legality. In R (Privacy International) v Investigatory Powers Tribunal, the relevant provision said: [‘Except as provided by virtue of s 67A, award, orders and other decisions of the Tribunal (including decisions as to whether they have jurisdiction) shall not be subject to appeal or be liable to be questioned in court’]. The wording in parentheses was likely inserted specifically to distinguish this provision from the ouster clause considered in Anisminic v Foreign Compensation Commission. A majority of the Court, however, nevertheless decided that that provision did not apply to ‘purported determinations’, including judgments where the IPT erred as to its own jurisdiction. Notwithstanding the fairly clear statutory wording, the Court read in a constraint to the provision.

Might an analogous argument have been offered on behalf of Ms Ammori? In the passage from Mr Justice Chamberlain’s judgment above, he notes that it may be legitimate to interpret a statute ‘more narrowly than its express language suggests’, but not to interpret it ‘contrary to its express language, read in context’. The ‘context’ referred to here is perhaps that other subsections in the same provision make explicit reference to violence against persons and to endangering lives. By leaving action that causes serious damage to property as a separate subsection, it might be argued that the legislature intended that section 1(2)(b) not be limited, for example, only to property damage that also endangers lives. This, however, would be a fairly flimsy textual basis for holding that such constraints cannot be read into section 1(2)(b). The point of the principle of legality is that if a provision is to be read as licensing interference with a common law right  in a particular context, that must be articulated in clear and express terms in the statute, or through necessary implication. In other words, if section 1(2)(b) is to be interpreted as applying to property damage that involves no possibility of harm to persons, then that must be provided for in clear and express terms or through necessary implication. Absent such clear and express authorisation, the courts are required to interpret the precise legal meaning of section 1(2)(b) by reference to common law rights and principles.

Moreover, if we expand our view of ‘context’ beyond the sub-clauses surrounding section 1(2)(b), there are compelling reasons to think that the enacting legislature never intended the 2000 Act to apply to direct action of the kind Palestine Action engages in. Nour Hadir points out that section 1(2)(b) has never been used previously to proscribe a non-violent group. She argues convincingly that the decision to proscribe Palestine Action effectively ‘lowers the threshold of seriousness that needs to be crossed before the state can legally ban all manner of disruptive political protest’. Daniella Lock has pointed out that the when the 2000 Act was debated in Parliament, then Home Secretary Jack Straw defended it on the basis that proscription power was a power that would only be used ‘when absolutely necessary’. The then-Home Secretary was responding to a question about whether Greenpeace would be covered by section 1. Clearly, then, some limitations on the power were foreseen at the time. It is more than arguable that a constraint should be interpreted into section 1(2)(b) in accordance with the principle of legality.

It is worth noting too that the courts have in several cases included a justificatory requirement, such as a necessity or proportionality constraint, as part of the principle of legality’s application. They have, for example, held that the principle of legality requires the courts to interpret statutes in such a way that they empower decision-makers to interfere with rights only to the degree necessary to achieve some legitimate aim. In R v Secretary of State for the Home Department, ex p Leech, it was held that the question of whether the relevant legislative provisions empowered the Home Secretary to create rules allowing for the screening of prisoners’ correspondence depended in part on whether the interreference with prisoners’ rights was proportionate to the aims sought. While the courts have been inconsistent in their application of this justificatory aspect of the principle of legality, it was employed more recently in the important case of R (UNISON) v Lord Chancellor. Applied to Ammori, the argument would be that the proscription power in the 2000 Act includes a justificatory requirement; for example, that an organisation may only be proscribed if its proscription is proportionate to the aims sought. While this would run into the usual difficulties of overcoming judicial deference in the national security argument, it at least seems ‘reasonably arguable’ that the proscription of a direct action group like Palestine Action does not satisfy such a test, such that permission should be granted for the argument to proceed.

Perhaps one might argue that the cases like UNISON and Privacy International are to be distinguished from cases like Ammori because only the former involve legislation that threatened the institutional role of the courts. While it is true that the courts have shown greater willingness to adopt broad statutory interpretations where their own role is threatened, it is not obvious why a violation of other individual rights should not attract a similar approach, if the purported violation were serious enough. It is also worth noting that the subsequent judgment in Privacy International’s chain of litigation makes clear that the principle of legality guides the proper interpretation not just of legislation concerning the role of the courts, but also legislation that threatens other important rights like privacy, even in the national security context. In its original case before the Investigatory Powers Tribunal, Privacy International sought a declaration that the Foreign Secretary’s practice of granting GCHQ ‘thematic’ warrants for computer hacking was ultra vires the powers granted in the Intelligence Services Act 1994. Privacy International argued that the relevant provisions must be read in accordance with the principle of legality, owing to the broad interference with property rights they were purported to license. Burton J, President of the IPT, was strikingly unsympathetic. After the Supreme Court determined that decisions of the IPT could be judicially reviewed, however, the matter went to the High Court. This time, the Court took the principle of legality more seriously than had the IPT, accepting the argument that the 1994 Act must be read consistently with common law principles, and noting: ‘The national security context makes no difference as otherwise the courts would sanction wide powers to override fundamental rights’ [21]. The principle of legality, then, can constrain the legislative conferral of powers to the executive in the national security context just as it can in any other context.

Returning to Ammori, given the fact that the use of the 2000 Act had never previously been used to designate an organisation as a terrorist group on the basis of damage to property that did not pose a threat to life or safety, there was an opportunity here for the Court to test the legality of that measure by considering the impact of common law principles on the proper interpretation of section 1(2)(b). Instead, the Court effectively reversed the principle of legality’s operation so that the interpretive burden worked to the advantage of the state: ‘If Parliament had intended such a restriction, it would have included it expressly. If it had done so, it would no doubt have defined “civil society or dissent group”, a phrase whose meaning is far from clear’ (IJ [65]). This is precisely the opposite to how the principle of legality works. The demand for legislative clarity is supposed to constrain the scope of the power that is granted to begin with; not operate as a means of ruling out restrictions on that power’s use.

Conclusion

Scholars of national security law have long criticised the breadth of the Terrorism Act 2000, and there is no doubt that the powers it confers are broad. But this does not mean that those powers come without constraints. In the context of Palestine Action, lawyers and scholars should not be willing to simply take for granted that the organisation falls under the definition of terrorism in section 1. That conclusion can only be reached after interpreting section 1 in light of the common law rights and principles impacted by it.

In Ammori, the High Court discouraged the applicant from pursuing the claim that the statute did not permit the Home Secretary to lawfully proscribe Palestine Action; instead funnelling that claim into an improper purpose argument that had less chance of success. Palestine Action’s case will proceed on different grounds. The judiciary would do well, however, to reflect closely on its own constitutional role in patrolling the legality of executive action, regardless of whether the executive invokes the context of national security as a prophylactic against legal oversight.

It merits reminder that since the proscription of Palestine Action, over 1600 people have been arrested for protesting the Home Secretary’s ‘decisions about how to protect the public from risks associated with terrorism’. If the proscription was unlawful, any subsequent arrests under sections 12 and 13 of the 2000 Act would also be unlawful. (Many of them are arguably unlawful even if one concedes that the proscription order itself was lawful. The extravagantly wide interpretation of ‘support’ for a proscribed organisation by the police in this context has been well-documented.) Against this backdrop, the importance of the Court giving proper consideration to the claim that the Home Secretary could not lawfully proscribe a group like Palestine Action to begin with can hardly be overstated.

I would like to thank Tanzil Chowdhury, Donal Coffey, Mike Gordon, Alan Greene, and Paul Scott for their generous comments on earlier drafts.

Conor Crummey is an Assistant Professor in Law at Maynooth University. He is author of The Principle of Legality: A Moral Theory (OUP 2025).

(Suggested citation: C. Crummey, ‘The Principle of Legality, the Definition of ‘Terrorism’, and Palestine Action’, U.K. Const. L. Blog (13th October 2025) (available at https://ukconstitutionallaw.org/))

---