By Alex Stevenson Follow @alex__stevenson
Cabinet ministers could force the SNP to hold a Scottish independence referendum "sooner rather than later", prompting anger from Holyrood.
The SNP's administration north of the border had wanted to hold a referendum in the second half of its parliament, but London may demand that the vote takes place within the next 18 months.
Prime minister David Cameron told Sky News this lunchtime that he wanted to end the "legal uncertainty" surrounding the referendum, but denied claims that ministers in London were going to "dictate" the terms of the vote.
Normally a referendum instigated by the Scottish government would be non-binding. Cabinet ministers are thought to be considering making the vote binding - and by doing so control its timing and nature.
A three-way question, which would include a form of extended financial devolution as well as the 'in' and 'out' options, had been thought to be favoured by the SNP.
Such an option could have a real chance of winning - but could be blocked by Conservative and Liberal Democrat ministers in London. Cabinet is discussing the issue as it meets at the Olympic Park in east London this morning.
Scottish deputy first minister Nicola Sturgeon said the "attachment of conditions" revealed that Westminster was meddling in Scotland's right to choose its future.
"Perhaps I should be relaxed about that because the more a Tory government tries to interfere in Scottish democracy then I suspect the greater the support for independence will be, but there is a key issue of democratic principle here," she told the Today programme.
"The SNP was elected on a clear prospectus and it's right that now that we have the mandate we can proceed on that basis."
All three mainstream parties in Westminster oppose the creation of an independent Scotland and had previously blocked calls for a referendum.
But the SNP won an improbable overall majority in 2011's Scottish elections, securing the democratic mandate to put the question to the Scottish people.
Outgoing Cabinet secretary Sir Gus O'Donnell warned that the question of "whether to keep our kingdom united" was one of the "enormous challenges" faced by Britain's politicians in the coming years.
Yesterday Mr Cameron called for a referendum to be "legal and binding" because the Scottish people "deserve some clarity, some decisiveness".
He accused SNP leader Alex Salmond of trying to engineer a situation where independence takes place by the back door.
"He knows the Scottish people at heart don't want a full separation from the UK and so he's trying to create a situation where that bubbles up and happens," he told BBC1's The Andrew Marr Show.
"I don't think we should just let this go on year after year. My view is sooner rather than later would be best."
Mr Salmond would prefer the referendum to take place in 2014 - the 700th anniversary of the Battle of Bannockburn in which Scottish forces inflicted a massive defeat on an invading English army headed by Edward II.
Scottish Labour's new leader, Johann Lamont, called for the referendum to take place as soon as possible.
"We want it to be run in Scotland," she said.
"If these proposals help there to be a quick, clear and decisive referendum result we would welcome them, but we have yet to see the details."