Harriet Harman, Labour's deputy leader, has slapped down figures in her own party who are hoping to pave the way for a Lib-Lab coalition by wooing the Liberal Democrats.
She told The Independent: "There will be no cosying up to the Lib Dems. No nods and winks. No political games. No hidden agenda aiming for a future coalition."
Ms Harman's intervention will be seen as a rebuke for Ed Balls, the shadow Chancellor. In an interview with this newspaper two weeks ago, he offered talks with Vince Cable, the Business Secretary, on a new Lib-Lab consensus, including his plan for a mansion tax on homes worth more than £2m. It has also emerged that Mr Cable exchanges friendly text messages with Ed Miliband. But Ms Harman said it would be "misleading" to claim the Liberal Democrats were better than the Tories. "It belies what they are doing," she said.
Ms Harman's move reflects concern in the Labour high command that the public flirting with Mr Cable sends a damaging signal to voters that Labour believes it cannot win an overall majority in 2015.
In a marked change of tone, Labour will go into attack mode against the Liberal Democrats, whose autumn conference in Brighton opens tomorrow. After Mr Clegg's dramatic mea culpa over his broken promise on tuition fees, Labour will challenge him to "put his money where his mouth is" by calling a Commons vote later this month on the Opposition's plan to cut the maximum fee from £9,000 to £6,000.
Labour will issue a dossier, What have the Lib Dems REALLY done for you?, setting out a long list of alleged broken promises and mistakes by Mr Clegg's party since it entered the Coalition, arguing that it must take responsibility for the Tory-led Government's failings.
Ms Harman insisted her party's tougher approach towards the Liberal Democrats would not change if they dumped Mr Clegg and installed Mr Cable, who has admitted his heart "beats on the left". She said: "We want everyone to be in no doubt. We have to judge people by what they have done. The Liberal Democrats have broken their promises. They are not a brake on the Tories. They are their accomplices."
Ms Harman declared that all Liberal Democrat ministers were culpable. "They are Cable actions as much as Clegg actions. It is how you vote that matters – not the mood music you make," she said. "It would not change the nature of the beast if the Lib Dems changed their leader."
Labour's deputy leader said the entire party – the Shadow Cabinet, National Executive Committee and Parliamentary Labour Party –agreed it should draw a line under speculation about a deal with the Liberal Democrats in 2015. "We are putting forward our policies and our arguments. We want to win an overall majority. We are not going to say the Lib Dems are a better option than the Tories. It would be misleading for voters," she said.
Some Labour figures disagree with Ms Harman. Neal Lawson, chairman of the Compass think-tank, said the party should prepare the ground for a deal: "We have to build relationships now. We might find we have more in common than we think."