CONSTITUTIONS tend not to offer guidelines for the break-up of the state.

It is unsurprising therefore the Scotland Act is largely silent on whether Scotland might leave the UK. After all, the point of devolution for the Labour Party was to make the Union stronger, not weaker.

The current dispute between the two governments revolves around different interpretations of what the Act does and, more importantly, does not say. The UK Government line is: The Union is a matter reserved to Westminster by schedule 5 of the Act. In terms of section 29 the Scottish Parliament has no power to pass law that "relates" to a reserved matter. Therefore a referendum on independence is at least legally questionable and quite possibly illegal.

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The Scottish Government replies the holding of referendums in general is not a reserved matter – the structure of the Scotland Act is such that matters not expressly reserved in schedule 5 are devolved and there is no such express reservation of the referendum power – and provided the referendum does not in itself purport to bring about secession unilaterally then it is within the Scottish Parliament's powers.

The latter position seems to be perfectly plausible. 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. We also need to take account of section 101 which is often missed in these debates. This provides that any Act of the Scottish Parliament which could be read to be outside the powers of the Parliament is to be read as narrowly as is required for it to be within competence, if such a reading is possible. It seems it would be possible to read legislation providing for a consultative rather than a binding referendum to be within competence using a liberal interpretation as invited by section 101. But we are at the early stages of this debate and the consultation exercise issued by the Scottish Government will allow these issues to be further explored.

The UK Government's current intervention seems to be less about this point of principle and more about regaining some control of the process issues that surround the referendum itself, in particular the timing of the vote, the setting of the question and a role for the Electoral Commission.

But here it has a problem. For Westminster to pass legislation on these issues would most likely, by constitutional convention, require the consent of the Scottish Parliament. But why would Holyrood trade a general power for a more restricted one? The outcome of a referendum generated from Edinburgh may be advisory only, but the political impact of a Yes vote would surely be irresistible.

l Professor Stephen Tierney is director of the Centre for Constitutional Law at the University of Edinburgh