I read with great interest the stimulating post written on this subject by Nick Barber. He makes three points that I would take issue with.

The first is a detail, but an important one none the less. Nick summarises the position of the UK Government as being that the UK Parliament can delegate to the Scottish Parliament the power to hold a referendum on Scottish independence but that it can also “impose limits on the exercise of this power”. It is important to point out that this is not what Her Majesty’s Government has proposed in their recently published consultation paper on Scotland’s Constitutional Future.  Rather, their proposal is that a section 30 order be made whereby clear legal authority is vested in the Scottish Parliament to pass an Act authorising a lawful referendum. A section 30 order requires the approval of each House of the UK Parliament and the approval of the Scottish Parliament: this is a matter of law and is not dependent on the Sewel Convention (see Scotland Act 1998, Sched 7). It is not therefore possible for the UK to “impose” anything on Scotland using the section 30 order procedure that the UK Government has proposed should be used in this case. It is worth spelling this out because Scottish Ministers have wrongly asserted in the past few days that the United Kingdom Government is seeking to dictate matters. It is doing no such thing.

The second point is more substantial. Nick asserts that the Scottish Parliament has “wide powers” and that it “can almost certainly run a referendum if it wishes”. Nick’s analysis of this point relies on a distinction between what he calls binding and advisory referendums. I return to this distinction in my third point, below. What is surprising about Nick’s analysis is the absence from it of the relevant law. The starting place, of course, is section 29 of the Scotland Act 1998. This provides that an Act of the Scottish Parliament (ASP) “is not law” if it is outside the legislative competence of the Parliament, and that an ASP (or a provision of an ASP) is outside legislative competence if “it relates to reserved matters”. Subsection 3 tells us that this issue is to be determined “by reference to the purpose” of the provision in question, “having regard (among other things) to its effect”. Now, Schedule 5 to the Scotland Act reserves “the Union of the Kingdoms of Scotland and England”. Thus, an ASP that related to the Union would, quite clearly, not be law. Contrary to Nick’s stated view, it is not simply that the Scottish Parliament cannot “pull Scotland out of the Union”: it is that any provision of an ASP that related to the Union would be “not law” within the meaning of section 29. Would an ASP authorising a referendum on Scottish independence be one that related to the Union? Of course it would.

Lest there be any doubt about this, let us develop the argument by considering the matter of “purpose”. The purpose of the Scottish National Party in promoting a referendum on Scottish independence could not be clearer. On page 28 of the SNP’s manifesto   for the May 2011 Scottish parliamentary elections, it is declared that “A yes vote will mean Scotland becomes an independent nation”. The purpose of the referendum for which the First Minister claims his famous mandate is unambiguously to deliver independence. It is not to begin negotiations with a view to exploring the possibility of rethinking the Union. It is to deliver independence: to break the Union, to break up Britain. Applying the purpose test of section 29(3) of the Scotland Act therefore only reaffirms the result already arrived at. That is to say, the Scottish Parliament clearly lacks the requisite legislative competence.

Finally, some words about “binding” versus “advisory” referendums. There has been a great deal of confusion about this matter this week, not least in several remarks of the Scottish Ministers. Two quite different things are meant by this distinction. The first relates to the legal effects of any referendum. Most referendums in the UK are advisory in this sense. That is, the decision taken in a referendum does not, in and of itself, effect a change in the law. Some other intervention or mechanism is required to achieve this, such as a legislative amendment or some form of ministerial order. Of course, a referendum could be set up such that its result would be legally binding: the statute authorising the referendum could make provision to this effect. As Nick reports, this was done in the case of the AV referendum, in the Parliamentary Voting System and Constituencies Act 2011, section 8. This sense of the binding/advisory distinction makes no difference to the question of the legislative competence of the Scottish Parliament to hold a referendum on Scottish independence. Whether or not the ASP purporting to authorise such a referendum contained a provision equivalent to section 8 of the PVSC Act, the matter would be outwith legislative competence for the reasons given above.

Recall also the rather elementary point that something can be binding in the British constitutional order without it being legally required in the strictest sense. Referendums are not opinion polls: their purpose is not to test public opinion, but to make decisions. They are appeals directly to the people to make a decision that, for whatever reason, is felt to be more appropriately made by the public than by a legislature. As the House of Lords Constitution Committee observed in its authoritative report on referendums and their place in the UK constitutional order, even where a referendum was legally only advisory, “it would be difficult for Parliament to ignore a decisive expression of public opinion” (12th report of 2009-10, HL 99, para 197). This sense of the binding/advisory distinction may therefore not amount to very much.

But there is a second sense attributed by some commentators to the binding/advisory distinction. In the Herald on 11 January 2012, for example, Professor Stephen Tierney argued as follows: “If a question is carefully crafted, asking people whether or not their preference is for independence and making clear this would only be treated by the Scottish Government as a political mandate to enter negotiations, this would seem to fall within competence”. This is an echo of the argument made by the late Professor Sir Neil MacCormick, one of the SNP’s most significant figures in the late twentieth century: see, for example, his essay “Is there a constitutional path to Scottish independence?” (2000) 53 Parliamentary Affairs 721, at pp 725-6. A positive answer to such a referendum question, it is claimed, would do no more than authorise the Scottish Government to negotiate with the United Kingdom Government. The problem with this is twofold. First, a bespoke mandate to renegotiate the terms of the Union, let alone to negotiate its dissolution, is clearly a mandate that “relates to” a reserved matter and falls foul of section 29. But moreover, the Scottish Government already has what MacCormick described as “unlimited powers to negotiate with the Westminster government about any issues which could be the subject of discussion between them”. There is no need for a referendum to be held before the Scottish Ministers can talk to Her Majesty’s Government about Scotland’s constitutional future. Indeed, such discussions have been ongoing throughout the prolonged passage through the UK Parliament of the current Scotland Bill. Thus, and to return to the words of section 29 of the Scotland Act, it could hardly be held to be the purpose of such an “advisory” referendum simply to authorise negotiations. No, the purpose of the referendum would not be to authorise an inter-governmental conversation. Its purpose would be as set out in the SNP’s own words (in its 2011 manifesto), that “Scotland becomes an independent nation”.

Professor Adam Tomkins is the John Millar Professor of Public Law at the University of Glasgow. He writes here in a purely personal capacity.