David Cameron has suffered "utter humiliation" over the nomination of Jean-Claude Juncker as European Commission president, Ed Miliband has claimed.
The Labour leader told MPs the PM's renegotiation strategy for the UK in Europe was now "in tatters".
But the prime minister insisted he would work with Mr Juncker despite his opposition to him.
He accused Mr Miliband of being "opportunistic and wrong".
The two leaders clashed during heated Commons exchanges following Mr Cameron's statement on the European Council gathering last week, at which EU leaders chose Mr Juncker.
Mr Cameron forced a vote of EU states on Friday on the selection of the Luxembourg politician - but lost it by 26 to two.
Mr Miliband mocked the prime minister for being unable to get other countries to support his stance.
"You were outwitted, out-manoeuvred and out-voted," he said. "Instead of building alliances in Europe, you've burned them. You're a defeated prime minister who can't deliver for Great Britain."
But Mr Cameron hit back that Mr Miliband's performance was "worthy of Neil Kinnock", the former Labour leader who lost two general elections.
"The fact is that the leaders of the principal parties in Britain agreed that this person was the wrong one, but as soon things get difficult, the weak give up the chase."
Defending his actions, he said he felt it was wrong that the European Parliament effectively dictated the choice of the new president of the commission and it "was important to push the principle and our deep misgivings about this issue right to the end.
"I at least wanted to put Britain's opposition to this decision firmly on the record."
While Mr Juncker's nomination marked "a bad day for Europe", he said the UK must now work with him, while also being "the voice" of those seeking change in Europe.
Deputy Prime Minister Nick Clegg, who was not present in the Commons for Mr Cameron's statement, told Mumsnet any notion of Britain leaving the European Union would be "immensely damaging".
But the Lib Dem leader added that it was now important to "move on" from the debate over Mr Juncker's appointment to "secure Britain's place permanently in the European Union".
UKIP MEP Diane James accused Mr Cameron of "blundering from one desperate act to another", saying that he, the European Commission and the voters all knew "that the big EU policies that affect the UK are simply not up for grabs."
Earlier on Monday, Joaquin Almunia, vice-president of the European Commission said "it would be very bad news" if the UK left the EU.
He also predicted that Mr Cameron would be able to work with the in-coming European Commission president, Jean-Claude Juncker.
He told the BBC Mr Juncker was "a committed pro-European" but also "a pragmatic politician" and the UK "was an important member of the EU".
'Benefit of the doubt'
Only Britain and Hungary voted to block the appointment of Mr Juncker, who is seen as a backer of closer political union in the EU.
Although Germany was on the opposite side over Mr Juncker, its finance minister told the Financial Times a British exit from the EU was "unimaginable" and "absolutely not acceptable".
Wolfgang Schauble said his country would do everything in its power to keep Britain in the union
"Clearly, we have in many economic questions and regulatory questions a broad consensus," he said.
"Historically, politically, democratically, culturally, Great Britain is entirely indispensable for Europe."
The Labour ex-European Commissioner Lord Mandelson, who met Mr Juncker in Berlin last week, told BBC Radio 4's Today programme that he should be given "the benefit of the doubt".
"He explicitly said he does not advocate a united states of Europe - he's not a green-eyed federalist minister as some in Britain have portrayed him.
"Mr Juncker has the experience and the knowledge to be an effective president of the European Commission."
But prominent Conservative Eurosceptic MP David Davis said while Mr Juncker was a "bad candidate", Mr Cameron had to turn his opposition to the EC president into a "tactical advantage" to secure constitutional changes that allow Britain to protect "our national interests".