Ed Miliband would find it "difficult" to work with Nick Clegg in government after the 2015 general election, the Labour leader has said.
His comments continue a flurry of unwanted attention over the weekend on the deputy prime minister's leadership prospects after the end of the coalition in three years' time.
Clegg had told the People newspaper that he was prepared to do his "duty" and enter into a fresh coalition with Miliband in a hung parliament scenario with Labour as the biggest party.
The comment was seen as a direct riposte to business secretary Vince Cable's refusal to rule out his own candidacy for the Lib Dem leadership should a vacancy ever arise.
"Clegg's biggest problem is that he will say he is a brake on the Tories, but he is an accomplice," Miliband said in an interview with the Independent newspaper.
Referring to a meeting with David Cameron after the alternative vote referendum defeat of May 2011, in which the prime minister offered Clegg his choice of policy priorities, Miliband added: "He chose not to kill the health and social care bill, a really bad bill doing damage to the NHS, and to pursue House of Lords reform."
Labour loyalists will approve of their leader's approach as it inverts Clegg's own refusal to work with Gordon Brown during coalition negotiations after the 2010 general election.
"I would find it difficult to work with him," Miliband said of the current Lib Dem leader.
The leader of the opposition had previously said he has no personal "animus" with Clegg and the pair even met to discuss cooperation on issues where their two parties agree in January 2011.
The relationship appears to have deteriorated since then, however. Even on issues where Labour and the Lib Dems agree in the current parliament there are now few signs of cooperation.
Labour has declared its support for Lords reform but opposed the crucial programme motion that would have smoothed the bill's passage through the Commons.
That allowed Conservative backbench rebels to guarantee a defeat for the government, handing Clegg a major setback in his bid to achieve a mainly elected second chamber.
Miliband's position on the legislation remains unclear. He called on the government to "get on with it" and warned that there remains "a huge distance to travel" before cross-party consensus is achieved.