Nick Clegg has stepped up his assault on David Cameron's plan to reclaim powers from Brussels, branding it a "false promise wrapped in a union Jack".
The Deputy Prime Minister said other EU states would never allow Britain to "pick and choose" the best bits of the grouping. He also suggested the idea was being used as a way of "unravelling" the UK's relationship with Europe altogether.
The intervention, in a speech at the Chatham House think-tank, risks further enraging Tory backbenchers who inflicted a stinging defeat on the coalition over the EU budget on Wednesday night.
Chancellor George Osborne attempted to ease tensions after the Commons demanded a real-terms cut in the seven-year funding package. He admitted frustrations among the Conservative rank-and-file were "understandable", and reiterated that David Cameron would only strike a deal at a crunch EU summit next month if it was "good for the British taxpayer".
But the Liberal Democrat leader was defiant, insisting Labour and the 53 Tory rebels had to "grow up" and act in the national interest. He said in an ideal world he would prefer a reduction in the EU budget, but the Government could not wave a "magic wand".
Mr Clegg suggested getting agreement on an inflation-only rise from all 27 countries would be a major coup, and if that happened Parliament would be asked to confront the "real hard choices".
Scuppering the deal in the quest for a cut would merely result in annual budget-setting, which was likely to be more expensive and the UK could not veto it, he said.
"Then I think it will be up to, particularly, the Labour Party to decide whether they are going to grow up, stop playing these playground games in Parliament and show that they are capable of making mature decisions in the national interest," he added.
Mr Clegg also indicated that he would support Mr Cameron if he wielded the veto on a deal that was not in the country's interests. "There is not a cigarette paper between myself and the Prime Minister on this issue," he added.
Tory ministers including Mr Cameron have suggested that fundamental reform of the eurozone could present an opportunity for Britain to loosen ties in areas such as social and employment law. Foreign Secretary William Hague has launched a full-scale review of how EU legislation affects the UK.