OPEN warfare broke out last night between the Coalition parties as senior Scottish LibDems dug in over plans to vote against David Cameron in the boundary-changes dispute, accusing him of losing his authority.

The party's chief whip in the Commons, the Orkney & Shetland MP Alistair Carmichael, said there was "no prospect" of LibDem ministers being sacked if they voted against the Prime Minister's plans to redraw constituency boundaries, despite some Tory MPs insisting such "treachery" must be punished.

After a week of rancorous feuding between the parties, he also cast doubt on the LibDems being able to work with "an unreconstructed Conservative Party" in a potential second Coalition term.

Meanwhile Scottish LibDem leader Willie Rennie said Cameron appeared to have lost his authority and had a "leadership issue" he needed to fix.

Under Cameron, the days of iron discipline among Tory MPs had "long gone", he said, prompting an acid response from Scotland's sole Tory MP about the LibDems' "disastrous poll ratings".

The "tit-for-tat" row was sparked by Tory MPs refusing to accept a LibDem proposal to reform the House of Lords, forcing Nick Clegg to admit his cherished project had collapsed.

In retaliation, he said LibDem MPs would help vote down Cameron's plans to redraw Westminster boundaries and cut the number of seats by 50 to 600, a change which could have helped the Tories gain an extra 20 MPs at the next election.

Infuriated at the LibDems' move, which makes it harder for Cameron to win a majority in 2015, many Tory MPs have demanded that any LibDem minister who votes against boundary changes be sacked under the convention of collective responsibility.

However Carmichael said that ministers were only sacked if they voted against Government policy, and Government policy had to be agreed by both sides, which the boundary changes were not.

He said: "There is only Government policy where the parties have agreed it. That is no longer the case here. And accordingly, if it were to come to a vote the entire Liberal Democrat parliamentary party would be whipped to vote against."

Asked if the past week had made a future alliance with Labour more appealling, he said the Coalition was sound – but only because it had a unifying purpose.

"The reason that I think this Coalition is sound for the duration of the parliament is that it is held together by the glue of fixing the economy.

"Now, if you don't have that glue of fixing the economy, if you don't have common purpose, then you would have to find another common purpose.

"With an unreconstructed Conservative party, which I think we can now see we still have, then that common purpose is more difficult to build."

Praising Clegg's role, Rennie laid the blame for the past week squarely on Downing Street.

He told the Sunday Herald: "David Cameron's inability to lead his backbenchers through the voting lobby in support of government policy was the root cause for the collapse of Lords reforms.

"The contrast between David Cameron's leadership with that of Nick Clegg's was striking. Nick Clegg provides good, calm and clear leadership especially in a crisis.

"David Cameron seems to have lost authority. When I first got into politics the Tories were always admired for their discipline. Those days have long gone.

"David Cameron has a leadership issue that he needs to resolve and his backbenchers need to honour agreements reached."

Mike Crockart, the LibDem MP for Edinburgh West, said Lords reform was always going to be a thorny issue, but said it was proving more of a headache for the Prime Minister than Clegg.

"This is more an issue for David Cameron in that he can't control his party. It's not so much a coalition problem, it's a Conservative leader problem."

Scotland Office minister David Mundell, the sole Tory MP in Scotland, said: "These remarks are disappointing but come as no surprise given the Liberal Democrats' disastrous poll ratings.

"But Willie Rennie's colleagues in London agreed to sign up to the Coalition with the Conservatives in the best interests of this country and we will see that through.

"The agreement is for the long haul, but these remarks unfortunately are in the interests of short-term expediency."

Cameron's failure to get boundary changes would mean an uphill struggle at the next election.

John Curtice, Professor of Politics at Strathclyde University, said the past week was a turning point, not for the Coalition, but for the Tories.

"It has damaged the chances of the Tory party being in power after 2015," he said. "Assuming the boundary reviews now get voted down, the Tories need a 10-point lead [over Labour] rather than a seven-point lead to get a majority." He said the LibDems' failure to secure reforms to the Commons voting system last year and the Lords mean the party would be loath to enter a second coalition with the Conservatives.

"The current coalition is at least going to survive for the foreseeable future, but the chances of any LibDem leader being allowed by his party to do another deal with the Tories, given the Tories have basically scuppered both of the spoils of constitutional reform that they were hoping to get, are much diminished," he said.

"We are increasingly getting the impression that this is a one-term Government."