Nick Clegg is facing a Liberal Democrat rebellion after endorsing another two years of public spending cuts following the next general election.
Some Liberal Democrat ministers and MPs fear that the party's attempt to have a distinctive appeal at the next election will be wrecked by George Osborne's announcement in his Autumn Statement on Tuesday that planned spending will fall by £8.3bn in 2015-16 and £15.1bn in 2016-17.
Lord Oakeshott of Seagrove Bay, a close ally of the Liberal Democrat Business Secretary, Vince Cable, said: "We Lib Dems signed a five-year Coalition Agreement, not a seven-year one.
"If we are seen to be chained to the Tory mast past 2015, it will kill our chances of fighting the next election as a genuinely independent force, able to form a coalition with whoever the people decide."
The peer said "alarm bells are starting to ring" and recalled that the National Liberals who joined the Conservatives in a coalition in the 1930s were "hugged to death".
He added: "At the last election, 38 of our 57 seats were won against the Tories. In many cases, there had been quite a high Labour vote we had to squeeze. If Labour voters are not prepared to vote tactically for us next time, we are going to lose many of these seats to the Tories."
One Liberal Democrat MP said: "It seems we have been bounced by George Osborne into backing the cuts in the Tories' next election manifesto. This will make it harder for us to have an exit strategy from the Coalition."
The unexpected commitment to more cuts is an early sign of the difficulties the Liberal Democrats will have in the run-up to the 2015 election. Mr Clegg could face strong pressure from Liberal Democrat grassroots to keep the party's spending options open when it drafts its next manifesto.
Last night Liberal Democrat leadership sources played down the prospect of a rebellion. A party spokesman said: "As part of the Coalition Government, we are committed to a credible deficit reduction plan that, according to the Office of Budget Responsibility's forecast yesterday, will mean that the structural deficit is eliminated by 2017.
"The actions we have taken will keep the economy safe and protect people from rising interest rates. At the 2015 election, the Liberal Democrats will fight as an independent party that delivers economic credibility and greater fairness. The Coalition Government's autumn statement confirmed the spending totals for the first two years of the next Parliament. We have not as a Coalition Government taken the decisions on the detailed breakdown of spending in these years, or even when those decisions will be taken."
Danny Alexander, the Liberal Democrat Chief Treasury Secretary, could not say where cuts required after 2015 would fall. "In good time, well before the election, we will set out where those savings will be made," he said. Asked if the Liberal Democrats would go into the next election promising nearly £30bn more austerity, he replied: "I'm afraid so."