Ex-Conservative leadership rival Ken Clarke has warned that David Cameron risks becoming the Conservatives' most Eurosceptic leader ever.
Mr Clarke called the decision "slightly head-banging"
The former chancellor urged Mr Cameron to drop his plans to withdraw Tory MEPs from the centre-right European People's Party in the European Parliament.
He said it would be wrong for the Tories to be "waltzing off" to find new "ultra-nationalist" allies.
Tory policy director Oliver Letwin played down Mr Clarke's attack.
Mr Clarke urged Mr Cameron to take more time over his plans for Europe.
He told the BBC: "I hope he will take many months over his decision in the European Parliament and perhaps decide that being a more extreme Eurosceptic than any of his predecessors is not the best way to launch himself on the international scene.
"Some of our really hard-line people apparently have persuaded him that he must break ranks and leave all these Christian Democrats, these Scandinavian Conservatives and Gaullists and people and start waltzing off looking for allies amongst the ultra-nationalist right in central Europe."
Mr Cameron took over as Tory leader on Tuesday
Mr Clarke told The Politics Show he hoped Mr Cameron would change his plans, but was worried the Tories new leader was too committed to the plan to turn back.
"Given all the goodwill that is surrounding his taking over and the optimism everybody has for him, I really think what a pity to insist on finding some new, slightly head-banging European Eurosceptic decision to take up as his first act in the leadership," he said.
Mr Letwin said Mr Cameron would not change his mind: "There may be some criticism, but David Cameron is determined that we should do this, not because it is the most important thing in the world, but because it is important that we are consistent.
"It is important that what we say at Westminster, how we act there, is the same as what we say and do in Brussels."