London and Edinburgh appear to be heading towards a major constitutional row over the date of a Scottish independence referendum, after the coalition government declared that Scottish First Minister Alex Salmond couldn't lawfully hold a vote on the matter himself.

After taking legal advice the government in London believes that matters pertaining to a breakup of the UK are "reserved matters", and as such remain under the jurisdiction of the Westminster parliament.

Westminister is considering giving greater powers to the Scottish Parliament so it can hold a referendum on independence, but the conciliatory gesture comes with a sting in the tail - the government in London wants to only change the law if Salmond agrees on a date by which the referendum would be held. The latest gamble by David Cameron risks antagonising nationalists in Scotland, as well as raising concerns among unionists.

Shortly after the government announced the current unlawfulness of Scotland unilaterally holding its own referendum, Scotland's Deputy First Minister Nicola Sturgeon announced that the SNP wanted to hold a vote on independence in autumn 2014.

A consultation process now underway is likely to spark tensions between London and Edinburgh over when the vote will be held, with David Cameron expected to come under pressure to allow the SNP to have the referendum powers alongside a polling date of its own choosing.

But on Tuesday evening Scottish Secretary Michael Moore hinted the coalition would push for an earlier vote as a condition of changing the law, saying while he would "not stand in the way of a referendum on independence", neither would he "stand on the sidelines and let uncertainty continue".

The Westminster government clearly believes it has leverage over the SNP in attempting to get the referendum held as soon as possible, long before the SNP's preferred date of 2014.

Michael Moore told the Commons: "As a Scot, I think it is vital that the Scottish people make a clear decision about our future with the United Kingdom, a decision made in Scotland by the people of Scotland.

"But at present there is a lack of clarity about the referendum, its outcome, and what the implications of that outcome would be, all of which creates economic uncertainty. That is bad for jobs and investment.

"This is not an issue that can be ducked. To legislate for a referendum on independence the Scottish parliament must have the legal power to do so. It is the Government's clear view that the Scottish parliament does not have that legal power."

Another caveat in the coalition's plan to resolve the legal deadlock is that there can only be one question - with a simple "yes/no" answer - on any referendum ballot paper, and any separate referendum on merely introducing greater devolution for Scotland couldn't happen on the same day.

See also: The background behind David Cameron's high-stakes gamble

The latest step on the path towards a referendum suggests behind the scenes the two governments have been unable to reach an agreed position. The disclosures today can be seen as the London government acting unilaterally to head-off a potential constitutional crisis and take the initiative on getting an early referendum.

If Scotland attempted to hold a referendum unilaterally under the current legal framework, it would have no legal weight whatsoever, the equivalent of an ordinary opinion poll.

David Cameron escalated his impasse with Alex Salmond on Sunday when he called for a referendum to be held within 18 months, claiming further delays would damage the economy.

Whitehall believes holding a referendum with more than two potential answers on the ballot paper would muddy the waters of an independence vote, which it considers too important to be considered alongside anything else.

The Westminster government has climbed down from a previous requirement that a referendum should be happen within eighteen months, but in effect is using claims that the law needs to be changed to insist upon a deadline. Currently no date for a referendum has been suggested, but the government at Westminster is indicating that they would still like it to be held as soon as possible.

Ministers in London say a deadline for any referendum would be decided in consultation between the two governments in the coming months, in a distinct blurring between the legal and the political dimensions to the controversy.

SNP sources believe that the decision to attach a deadline for the referendum to a change in the law came at the behest of George Osborne, who is chairing a Cabinet committee on Scotland's constitutional future.

Scottish National Party MPs at Westminster are clearly delighted at the latest impasse, believing it is causing a surge in new members joining the SNP in Scotland.

Scottish Liberal Democrat leader Willie Rennie has said it is important the future of Scotland is kept out of the courts.

“To avoid the strong possibility of a legal challenge some cooperation with Westminster is essential so that a referendum has the legal force and effect necessary to be seen as legitimate," he said.

“We do not want a situation to arise when this issue of vital importance to the future direction of Scotland is finally decided in the courts rather than at the ballot box.”

The coalition government admits it is in untested territory, but believes a referendum could be held as early as a year from now, but some legal changes will need to be made first.

Ministers have drawn up what's called a Section 30 Order, which would allow Scotland to hold a referendum without asking Westminster for permission, however this order needs to be agreed by both the parliaments in London and Edinburgh first.

The disclosures suggest that the Scottish National Party's manifesto pledge last year to hold a referendum had no basis in law, because the terms drawn up in the creation of the Scottish Parliament did not give the Scottish government powers to hold one.

The Scottish Office says any referendum on independence would have the specific purpose of intending for Scotland leaving the union. The government believes this is the case because the SNP government have made it clear they want this to happen. Because of this the government in London believes it must be a "reserved matter".

The government in London is not interested in whether any referendum would be merely advisory or legally binding. It believes that unless the Scottish Parliament's powers are changed, no referendum would be considered legal.

The Scottish Advocate General will address a meeting of lawyers in Glasgow next Friday to outline his legal position relating to these changes.