Shadow chancellor Ed Balls has claimed that it would be impossible for Labour to work with a "discredited" Nick Clegg following the next general election.
Mr Balls also accused the Deputy Prime Minister of justifying the Liberal Democrat U-turn on the economy by blaming Labour for the need to make spending cuts.
In an interview with The Guardian, the shadow chancellor said: "Clegg looks an increasingly desperate, shrill and discredited politician, losing both public and party support.
"People think that if Clegg says something, it cannot be the truth. The Liberal Democrats need to have some real hard thinking about what they stand for."
Mr Balls explicitly stated his party would not be able to govern with Mr Clegg after the next election.
"I don't see how Nick Clegg could change direction again with any shred of credibility, or how he could work with Labour now, but that is not true of Liberal Democrats more widely," he said.
The MP also warned that David Cameron was hoping that trade unions would stage a growing number of strikes, so he can claim that Labour is attempting to drag Britain back to the 1980s.
"The Labour movement mustn't walk into Cameron's trap," said Mr Balls.
"He wants to have that confrontation so that he can blame a difficult year, or even longer, on the trade unions and Labour taking Britain back to the 1980s. But it is David Cameron and George Osborne who want to take us back to the 1980s."
The shadow chancellor said it is right that people demonstrate and march, but wanted to make sure Mr Cameron was exposed "as the man that is taking us back to the confrontation and division that many people say scarred our country".
