A cabinet minister will warn Labour tomorrow that it faces the prospect of a hung parliament after the next general election.
Peter Hain, the Secretary of State for Northern Ireland and Wales, will urge Labour to turn its guns on to the Liberal Democrats, accusing the third party of conspiring with the Tories to push Labour out of office.
Mr Hain is the first minister to raise the prospect that Labour may lose its overall majority. Academics believe there is a stronger possibility of a hung parliament than for many years if the Tories maintain their recovery under David Cameron. They predict that a Tory lead of between one and 12 points over Labour in the share of the vote would leave no party with an overall majority.
Addressing Labour's South-west regional conference in Weston-super-Mare, Mr Hain will acknowledge that the Liberal Democrats' win in the Dunfermline by-election last month shows that they remain an important part of the political landscape. "We must take on the Lib Dems much more effectively at local level: don't ignore them but beat them at their own game," he will say.
The minister will say the significance of the by-election result was the "double gain" for the Tories. "With each seat lost our majority goes down by two, and we are now two seats closer to a Conservative government. As in the general election, votes for the Liberals let in the Tories, and Labour voters cannot consider the Lib Dems a safe protest vote," he will argue.
Mr Hain will warn that an "unholy alliance" between Liberal Democrats and Tories could be an even bigger threat to Labour by the time of the next election. He will claim Mr Cameron has hit a plateau and the Tories have an unspoken strategy to pitch for a hung parliament and a Tory-Liberal Democrat coalition.
"Equally, the Liberals' anti-Labour strategy means that their best bet is a hung parliament and joining David Cameron in a coalition," he will say. "Votes for the Lib Dems risk giving the Conservatives a chance to undo all the achievements we have fought for."
Mr Hain will say: "The Cameron camp know they would be defying political gravity to win outright next time: they are coming from too far back, and they don't have the lift-off they need. Indeed they have struck a plateau in the polls nowhere near high enough for the huge swing they need."
He cited comments by Kenneth Clarke, the former cabinet minister heading Mr Cameron's policy review on democracy, who said that he could envisage Sir Menzies Campbell, the Liberal Democrat leader, in a Cameron cabinet.
Mr Clarke told The Spectator magazine: "If [Gordon] Brown fights an election that produces a hung parliament, the public will think it his duty is to leave No 10 and accept defeat. And people will expect the Conservatives and Liberals to form a working government.
"The Liberals really would look as if they'd ignored the public's message, either refusing to play ball with anybody or helping a defeated Labour Party back into office. I'm glad to say the fates could condemn the Conservatives and the Liberals to form a coalition."