The Scottish people should have the chance to take part in a legally binding referendum on independence but that should happen within 18 months, the prime minister has indicated. Any question on independence put to referendum should be simply "yes or no" with no third option that could give Holyrood additional powers while maintaining the historic union, he suggested. The coalition will shortly publish its legal guidance supporting its strategy. He told the Andrew Marr programme yesterday:



Let's not drift apart. I think he [Alex Salmond] knows the Scottish people at heart do not want a full separation and so he is trying to create a situation where that bubbles up and happens whereas I think we need some decisiveness so we can clear up this issue. I don't think we should just let this go on year after year... My view is that sooner rather than later would be better.

The SNP has reacted with fury accusing Westminster of trying to dictate the future for the Scottish people, despite the fact that the coalition in Westminster is Tory-led and there is currently only one Conservative MP in Scotland.

Nicola Sturgeon, Scotland's deputy first minister, told the Today programme this morning (audio):

This is a blatant attempt to interfere in the decision that is really one for the Scottish Government in terms of the timing of the referendum and for the Scottish people in terms of the outcome. We were elected on the basis of our commitment to have a referendum in the second half of this parliamentary term. This is about Westminster seeking to interfere. We said very clearly in the election that our immediate priority if elected would be to seek to strengthen the Scotland Bill, currently going through the Westminster Parliament, to give the Scottish Parliament important economic job-creating powers, a referendum on independence in the second half of this parliamentary term... The decision on the future of Scotland should be for the Scottish people. That's democracy.

But what is the legal basis – who does have the right to set the conditions of the referendum, Holyrood or Westminster?

The question

Does David Cameron have the power to dictate when Scotland votes on independence?

I'm going to outline the political tactics at play here and talk to constitutional experts about legal position. Do you have any information that can help decide the issue? Get in touch below the line, email me polly.curtis@guardian.co.uk or tweet @pollycurtis

Analysis

The SNP said at the election that they would hold an indicative referendum on independence in the latter part of this parliament and not before the end of 2013 – however the timing was not explicitly set out in its manifesto.

However, constitutional experts say that a legally binding referendum would require Westminster to amend the law. Dr Cormac Mac Amhlaigh, an expert in constitutional law at Edinburgh University, told the Scotsman that the Scotland Act which established the Holyrood government specifically retained all constitutional matters in Westminster. Similar evidence has also been submitted to an inquiry currently being conducted by the Scottish affairs select committee in the Westminster Parliament, which among others things is investigating the legal responsibilities for a referendum.

Section 29 of the Scotland Act specifically retains constitutional issues including those affecting the union in Westminster.



29.— Legislative competence (1) An Act of the Scottish Parliament is not law so far as any provision of the Act is outside the legislative competence of the Parliament. (2) A provision is outside that competence so far as any of the following paragraphs apply— (a) it would form part of the law of a country or territory other than Scotland, or confer or remove functions exercisable otherwise than in or as regards Scotland, (b) it relates to reserved matters … And Paragraph 1 of Schedule 5 to the Scotland Act states that: The following aspects of the constitution are reserved matters, that is— (a) the Crown, including succession to the Crown and a regency, (b) the Union of the Kingdoms of Scotland and England, (c) the Parliament of the United Kingdom

Some lawyers have argued that this means that a referendum would have to open to the entire UK population and not Scotland. Aidan O'Neill QC made this argument in the Guardian at the end of last year.

However, the SNP says it could establish an indicative but not legally binding referendum without Westminster approval which, if they secured a yes vote, would be politically hard for Westminster to ignore. Professor Robert Hazell, the founder of the constitution unit at University College London and a former senior civil servant, told the Today programme this morning that the SNP had hoped to build support from an initial non-binding referendum in the latter part of the parliament but that Cameron had dramatically fast-forwarded those plans and in doing so could have potentially overstretched his powers.

Hazell said (audio):

The SNP government's plan had been that they would pass legislation sometime in the next couple of years to authorise a referendum but under the powers of the Scottish parliament that could only be limited to asking the people of Scotland to authorise the Scottish government to negotiate with the British government about independence. Going any further would be outside the powers of the Scottish government. The Scottish government has said they intend to hold a referendum with a question of that kind in 2014. There would then be negotiations with the British government about independence, step three. Then there would have to be step four, if people voted yes and the negotiations were successful the Westminster parliament would have to legislate passing a Scottish independence act because only Westminster can pass independence to Scotland.

He said that the PM's move was dramatic because he was essentially "fast-forwarding" to the end of that process.

It is a big gamble and I think the UK government has right on its side in insisting that any referendum is fair and legal and interestingly the Canadian government did exactly that ten years ago in an act of their parliament called the Clarity Act which provides that any referendum in Quebec has to be a clear decision on a clear question, that's following history in Quebec of two very ambiguous referendums in terms of the wording of the question. But whether the [Westminster] government's right to now itself set the timing of what is an SNP government policy and to insist that the referendum is decisive, there they may be over-reaching themselves.

Hazell is arguing that a legally binding referendum would indeed need Westminster's approval, but questioning whether to set the conditions or the timing and the way the form of question asked, vetoing a third option, could be over-stepping the mark. I'm going to try and find more evidence to test Hazell's suggestion and will update this blog as I do.

I've gone back to Aidan O'Neill QC, whose article I mentioned (above), to ask him about the legality question. He told me that ultimately the Westminster parliament remained the sovereign body after devolution and therefore it has the upper hand on constitutional matters – including the setting of the timing of a referendum:

From a legal point of view it seems it's entirely legal and the only legal way for a referendum to happen. There has to be action by the UK parliament. The Scottish parliament are saying its just a consultative referendum, you can only consult to change a power if they have the power to change it. The Scotland Act makes constitutional conflict into legal conflict. From a legal point of view there would be good grounds to challenge any action by the Scottish government on its own behalf. Even if it were not successful it would delay matters by two or three years. It's enough to mess it up massively. But ultimately the Westminster parliament is sovereign so can do whatever it wants and the Scottish parliament isn't.

Professor Robert Hazell, who I mentioned (above) suggesting that in attempting set the timing of the referendum Cameron could be "over-reaching" himself, has clarified his comments to me. He agrees that legally and constitutionally Westminster is within Cameron and the Westminster parliament's rights.

Legally Westminster remains sovereign. Westminster has reserved to itself quite explicitly that it can legislate on any matter including devolved matters and it's legislation trumps Scottish legislation. The constitution is item one in a schedule of reserved matters in the Scotland Act. There's no doubt about Westminster's powers legally and if the process reached the point where Scotland was being granted independence that could only be done by the Westminster parliament. The big question is how wise Cameron is politically.

Hazell said that the closest international parallel with Cameron's plans was in Canada where they established the Clarity Act to prevent a repeat of two ambiguous referenda on independence for Quebec. That said that any referenda requires a clear answer to a clear question. He points to an SNP white paper on independence which in 2007 (pdf) included a dummy question for a referendum, which read:

"The Scottish government should negotiate a settlement with the government of the UK so that Scotland becomes an independent state."



Hazell said:

Under the terms of the Clarity Act that would be fine. It's clear what the negotiations are about. The Canadian Clarity Act does not seek to impose a timetable, it leaves it up to Quebec and it doesn't say anything about a referendum needing to be decisive. I think what Cameron is saying, and this a very bold thing, that if the people of Scotland vote yes Scotland will become independent. That can only be authorised by Westminster. His challenge or offer to Salmond is OK, this is hardball, let's do this for real. He is hijacking the SNP's timetable. My advice to UK government would have been to sit on your hands, independence is an SNP policy and they have plenty of gates they have to get through. The likelihood is that the people would vote no in a indicative referendum – the British government needn't have got involved in the game at all. Salmond is a wiley operator, there is nothing he likes more than a fight.

He said that if Scotland went ahead unilaterally with a referenda, even if it isn't binding, it would be open to legal challenge, but that a challenge to a yes vote by Westminster would be unlikely as it would be a "red rag to a bull".

Privately, some MPs are already suggesting that Cameron's gamble is already that. One pro-union Labour MP has just told me that he thinks Cameron's game will simply harden support for a yes vote in Scotland.

Below the line several readers have also pointed out that this is a political argument, rather than a legal one. O'Neill said that the accusation that Cameron is attempting to stage-manage the referendum to engineer the no vote he wants by rushing it through with a question limited to a yes or no answer, could equally be levelled at the the SNP. Polls suggest that currently they would lose a referendum, though support for independence is on an upward trend. O'Neill also makes a more subtle argument that while the SNP can claim that as the ruling party in Scotland (which has only one Tory MP) they represent the people and therefore their timing of the referendum is justified, Cameron can claim to represent the majority unionists who voted at the general election (including Labour).

Both parties are accusing the other of being partisan about the constitution. Cameron is trying to fix it for his result; Salmond is doing the same. Salmond can say that he represents the people of Scotland; Cameron can reply that he represents Scottish unionists who won the majority of the seats in Scotland in 2010. But Salmond needs the UK government to come in for it to happen. There's a lot shadow boxing going on.

Severin Carrell, the Guardian's Scotland correspondent, has got in touch to point out that the SNP are arguing that constitutionally there is no such thing as a "binding" referendum as if it were binding that would challenge the sovereignty of the Westminster parliament. Salmond's office has circulated the following quotes to back-up its arguments for its own indicative referendum.

1. The leading textbook on Scottish Constitutional law, (Scotland's Constitution: Law and Practice, Himsworth and O'Neill, 2009) discussing the interpretation of section 29(3) of the Scotland Act [i.e. legislative competence] concludes:

A recurring hypothetical example with a high political profile is that of a Bill to authorise the holding of a referendum on independence for Scotland. Because its purpose could be interpreted as the testing of opinion rather than the amendment of the constitution, such a Bill would almost certainly be within the Parliament's powers.



2. Professor Alan Page, Dean of Law at Dundee University, Dundee Courier, 4th November 2011

My view is it was effectively settled by the parliamentary election in May. Once the SNP won an outright majority it was just accepted that there would be a referendum as a political fact. That, I think, puts an end to the arguments – there is going to be one.



3. Stephen Tierney, Professor of Constitutional Theory, Edinburgh Law School, Scotsman, 7 June 2011

The Scottish Parliament has authority to stage a referendum. The Union is a reserved matter under the Scotland Act; the Scottish Parliament certainly cannot unilaterally authorise secession. But a referendum question, carefully framed, asking the Scottish people if they would like the Scottish Government to enter into negotiations with the UK government, would seem clearly to be within the Scottish Parliament's competence.

4. Colin Munro, Professor of Constitutional Law, Edinburgh Law School, Scotsman, 11 March 1998

There is nothing to stop the Parliament arranging to hold a referendum, because that would not involve a change in the law.



5. Mark Walters, Associate Prof. of Law, Queen's University, Ontario, Modern Law Review 62 1999

The Scotland Act 1998 confers broad legislative powers which are (apparently) to be exercised on behalf of the Scottish electorate; hence, a consultative referendum – even on secession – would not conflict with the policy of the Act so long as its purpose is to assist the Scottish Parliament in determining the democratic will of the electorate.

Verdict

David Cameron has the law on his side if he wants to stage-manage a Scottish referendum on independence. The Scotland Act 1998 that established Holyrood also dictates clear limits to devolution: constitutional matters remain in Westminster, the SNP can't stage a binding referendum without Westminster's say-so and even an indicative poll to test the public opinion could be open to legal challenge.

However, the idea that the UK government would legally challenge a yes vote forcing the Scots to remain part of the UK shows the limits of Cameron's powers. He may have the law on his side, but the moral argument that Westminster should have the upper-hand in dictating the future for the Scottish people – particularly when there is currently only one Tory MP in Scotland – is shaky at best.

Constitutional experts said yesterday that while Westminster remains sovereign, Cameron has taken a huge political gamble by "hijacking" the SNP's plans to test public opinion in an indicative poll and fast-tracking a legally binding referendum.

Professor Robert Hazell, director of the constitution unit at University College London, said that according to international precedent – most relevant is the Canadian government's handling of referendums on the independence of Quebec – Cameron is within his right to argue for a clear question but could be "over-reaching" himself politically by dictating the timing and that the outcome should be "decisive".

Hazell said: "He [Cameron] is hijacking the SNP's timetable. My advice to UK government would have been to sit on your hands. The likelihood is that the people would vote no in a indicative referendum – the British government needn't have got involved in the game at all. Salmond is a wily operator, there is nothing he likes more than a fight."

If accusations of stage-management are to be levelled at Downing Street, they could also be applied to the SNP. Polls suggest that if asked today, the Scottish people would vote against independence, but that support is on an upward trend. The thinking behind the SNP's policy to hold an indicative poll in the latter part of this parliament is that by then the effects of coalition's spending cuts will start to pinch and Westminster and the union's popularity will wane. More time also means more time for them to sell the idea.

Aidan O'Neill QC, a senior Scottish lawyer, said: "Both parties are accusing the other of being partisan about the constitution. Cameron is trying to fix it for his result; Salmond is doing the same. Salmond can say that he represents the people of Scotland; Cameron can reply that he represents Scottish unionists who won the majority of the seats in Scotland in 2010. But Salmond needs the UK government to come in for it to happen. There's a lot shadow boxing going on."

Feedback

A brief round-up of some of the other issues that arose in comments from readers. Donald Macleod writes via email:

Under UN treaties (in particular the International Covenant on Civil and Political Rights, which was signed and ratified by Westminster) people have the right to self-determination. I'm not clear of the force in law of this kind of treaty, but it also provides for people to have the right to exercise self-determination. This would surely exclude any possibility of a UK-wide referendum, and it could be argued that if you have the right to exercise self-determination, it should be available to you on your own timescale, otherwise it's not a freedom at all. For the record I'm Scottish, pro-independence but not a member of the SNP - more aligned with the Greens.

I've not got a conclusive ruling on this, but a couple of counter-arguments I'd raise: the UN right is based on an ethnic people, which doesn't really apply to modern day Scotland. Self-determination doesn't necessarily mean independence, the Scottish Parliament could fulfil that UN obligation. Politically, however, I think it adds to the nationalist's argument.

Several readers have raised the case of MacCormich v Lord Advocate in 1953 which ruled on sovereignty in Scotland and England. Wikipedia has a summary of the case here. One anonymous reader writes in:

I've been following your blog today with interest, and I think an important point has been missed. The concept of parliamentary sovereignty is different in Scotland and England; in MacCormick v Lord Advocate (1953 SC 396), a case brought to challenge the right of The Queen to be styled Queen Elizabeth II in Scotland (as there has never been a QEI here), the Lord President of the Court of Session, Lord Cooper, (Scotland's top judge) gave his opinion that "the principle of unlimited sovereignty of Parliament is a distinctively English principle and has no counterpart in Scottish constitutional law". Also, "the Lord Advocate [representing the UK government] conceded this point by admitting that the Parliament of Great Britain 'could not' repeal or alter [certain] 'fundamental and essential' conditions" of the Act of Union".

Another point raised repeatedly in the blog comments and emails from readers is that sovereignty is different in Scotland because pre-union doctrine dictated that the Scottish people are sovereign. That sovereignty has never been tested vis-a-vis Westminster parliament's sovereignty. @Bellsider writes :

Under Article 19 of the 1707 Treaty of Union the independence of Scots Law and therefore Scottish Constitutional practice is protected for all time. In Scots law and constitutional practice, as pointed out by Lord Cooper in 1953, sovereignty lies with the people of Scotland and not the crown or parliament. Therefore neither the UK Parliament nor Hollyrood have the right to decide on the constitutional position of Scotland and the Scotland Act is subservient to this prior legislation. Since over 50% of the Scottish people voted for a referendum in the second half of this (SNP) governments term, then the SNP have the constitutional right to do so.

I asked Aidan O'Neill QC about this point via email and he wrote back:

The idea of the "sovereignty" of the Scottish people (usually contrasted with poor benighted English who, they say, only have a sovereignty of Parliament) is a romantic shibboleth but often trotted out by nationalists. It has no legal or contemporary constitutional meaning. It is part of an invented tradition which would highlight and consider to be constitutionally foundational the terms of the Claim of Right of 1689 when a self-appointed and assembled convocation of the Estates (Scotland's Parliament) dominated by the Presbyterian (anti-bishop) faction of the Reformed Protestant church in Scotland – declared that James VII of Scots (James II of England) had forfeited the Scottish Crown by virtue of his being a professed Papist. It is a profoundly sectarian anti-Catholic – and anti-Episcopalian document. If this is indeed the true voice of the Scottish sovereign people then a future independent Scotland would not be a great place for diversity to flourish.



I have written on all this at some length in an academic article: "Limited Government, Fundamental Rights and the Scottish Constitutional Tradition" (2009) Juridical Review 85-128.

@Ken900 claims that the notion of Scottish independence is more popular in England than Scotland. He writes:

Polls suggest that a majority of people in England support Scottish Independence.

Previously there has been some debate about whether the whole of the UK should be allowed to vote in a referendum. Interestingly, what Cameron has said so far seems to suggest that this is not an idea that sill be pursued.

Many thanks for all the comments on this blog. The standard of debate has been incredibly high and informed and has included suggestions of several other questions for me to pursue as this row rumbles on. Is a delayed referendum really bad for the Scottish economy? What impact for independence have for the Scottish and UK economies? Who gets to keep the debt if Scotland won independence? Any other suggestions do get in touch. Email me at polly.curtis@guardian.co.uk or tweet @pollycurtis