David Cameron and Ed Miliband's standoff over the voting rights of Scottish MPs is deepening, threatening a full-scale constitutional crisis which some are warning could yet imperil the union.
Defeated Scottish first minister Alex Salmond said he only lost the fiercely-contested referendum because of last week's 'vow' by David Cameron, Ed Miliband and Nick Clegg promising a drastic shift in money-raising powers to Scotland in the event of a 'No' vote.
Cameron now faces intense pressure from disgruntled voters, backbenchers and Cabinet ministers demanding that further Scottish devolution be accompanied by comparable changes across England.
The move has been backed by deputy prime minister Nick Clegg, but is opposed by Ed Miliband who has instead called for a constitutional convention addressing the issue next year.
Salmond told BBC1's Sunday Politics programme: "I am actually not surprised they are cavilling and reneging on commitments, I am only surprised by the speed at which they are doing it. They seem to be totally shameless in these matters.
"The prime minister wants to link change in Scotland to change in England. He wants to do that because he has difficulty in carrying his backbenchers on this and they are under pressure from Ukip.
"The Labour leadership of course are frightened of any changes in England which leave them without a majority in the House of Commons on English matters.
"I think the vow was something cooked up in desperation for the last few days of the campaign and I think everyone in Scotland now realises that".
With the attention of what Salmond has called the "Westminster political bubble" now moving away Scotland to the three party conferences, all sides are focusing on the importance of Labour's 40 Scottish MPs - who the Tories are hoping could be sidelined for votes on matters only affecting England.
With no party likely to win a majority of over 40 in next May's general election, the issue could prove decisive in determining its winner.
So after Conservative chief whip Michael Gove told the Times "it would be impossible to move forward without making sure you have change both in Scotland and England", Labour was quick to respond by attempting to kick the issue into the long grass.
Former prime minister Gordon Brown, continuing his unexpectedly forthright role in setting out timetables for devolution, gave a speech in Fife demanding that the Westminster leaders' 'vow' be kept.
"The eyes of the world have been upon us and now I think the eyes of the world are on the leaders of the major parties of the United Kingdom," he said yesterday.
"These are men who have been promise-makers and they will not be promise-breakers.
"I will ensure as a promise keeper that these promises that have been made will be upheld."
But Miliband's call for a constitutional convention, which would not be completed before next May, endangers the timetable for swift reform he promised last week.
The Labour leader insisted he would not "allow this moment to be used for narrow party political advantage".
And speaking on BBC1's The Andrew Marr Show today, he said: "We've spent two years trying to keep our country together. Let's have a proper constitutional convention, let's look at the issues, but let's not drive our country apart.
"I am in favour of one House of Commons with 650 MPs because we've fought tooth-and-nail over the last two years to avoid our parliament being split up."
Miliband's determination to avoid paying a political price for Scottish devolution endangers consensus, but he is keen to transfer responsibility for the clash to the prime minister.
"People right across the country are going to say David Cameron made a promise, he didn't make a conditional promise, and he's going to be kept to that," he added.
"Unless the establishment reacts to this wake-up call about how our country is run, how our economy is run, we are not going to address the discontent in England, Wales, Scotland and the whole of the United Kingdom."
Cameron posted a lengthy status update on his Facebook page this morning, arguing that "this moment" had to be about "settling other questions whose time has come".
"The challenge is to make sure our UK works for all nations," he argued.
"Millions of people in the rest of the UK have been listening to these debates, watching this campaign and rightly asking: 'What will change for us? Why can't we have the same powers and the same rights as those in Scotland?"
His party's chairman Grant Shapps, responding to Miliband's comments this morning, said the Labour leader had "failed to answer a very simple question of fairness – does he believe only English MPs should vote on laws that only affect England?"
In Scotland, the unravelling of last week's 'vow' is being greeted with a mixture of horror and denial.
Better Together campaign chief Alistair Darling told Marr the agreement was "non-negotiable" and warned that "anyone who welches on that will pay a very heavy price for years to come".
He added: "If anyone attempts to get out of that, how will anyone be believed on what they've got to say?"
The rushed nature of Cameron's promise may yet prove to be one of his biggest political errors. He is meeting Tory MPs at Chequers today in an attempt to find a way out of the crisis.
But the prime minister risks alienating voters as well as his political colleagues if he goes ahead with fulfilling his pledge to Scotland.
A poll of English and Welsh voters by Survation for the Mail on Sunday found 65% backed banning Scottish MPs from voting on English laws at Westminster.
Seven out of ten said they did not support the Barnett formula which gives Scots £1,600 more per head than the English.
And 59% voiced their support for an English parliament, with only 11% opposing it.
Justice secretary Chris Grayling is the first Cabinet minister to attempt to exclude Scottish MPs from voting against his department's controversial changes.
"In Scotland justice is wholly devolved," he wrote in an article for the Sunday Telegraph newspaper.
"As secretary of state and lord chancellor, I have virtually no role there. But Scottish MPs today can still vote on my proposals to toughen the justice system in England and Wales. That clearly cannot continue."
Clegg has suggested Cameron was coming close to reneging on the 'vow' because of the threat from Nigel Farage's Ukip.
"If the Conservatives enter into a Dutch auction with Ukip over ever more extreme solutions to the issue of English votes they could jeopardise the union they purport to defend," he wrote in an article for the Sunday Times.
"Surely we haven't fought to save our union in a vote north of the border, only to see it balkanised in Westminster?"