David Miliband and William Hague have clashed over the implications of the Conservative Party's European policy.
The foreign secretary warned on Friday that other EU governments are "tearing their hair out" at the prospect of a Tory government after the next general election.
He claimed it would lead to years of disputes over institutional reforms.
Miliband told BBC Radio 4's Today programme: "What William Hague is promising is a return to institutional wrangling and renegotiation of Britain's relationship with the EU if the Lisbon treaty has come to pass. That would isolate the UK in Europe.
"I go round Europe talking to centre-right governments as well as centre-left governments and they tear their hair out about the prospect of a Conservative government coming back to Europe after a general election and trying to renegotiate Britain's relationship with the EU. They think that will be bad for Europe and bad for Britain.
"If you care about Britain's influence around the world - and notably with the US - the idea that we are going to withdraw to the margins of Europe and increase our influence is absolute nonsense."
But the shadow foreign secretary rejected the claims, saying the Conservatives would "still be working in many ways" with other European parties, adding that "we have close relationships with them".
"When it is necessary to out-vote the socialists in the European Parliament, then the new grouping and the EPP will be working together in order to do that," he said.
"It is not true that the Conservative Party would retreat to the margins of Europe.
"Those centre-right governments... are not only willing to work with us, they are already working with us behind the scenes in anticipation of the demise of this dreadful Labour government in London."
Miliband insisted that Labour MEPs would "go to Brussels and start to work on issues of energy, climate change, migration and foreign policy".
"If you elect Conservative MEPs, they will be frog-marched out of the mainstream grouping of European conservative parties into an isolated position on their own on the edge of the European Parliament and that will be a metaphor for what a Conservative government would ensure if it became the government of Britain," he said.
But Hague responded: "The choice in this election is between Conservative MEPs who have the strongest record on transparency, who support a referendum on the Lisbon Treaty, who want to cut the administration costs of EU legislation and oppose wasteful spending throughout the EU, in contrast with a Labour record on Europe where our influence in Europe has not been extended, the cost of being in Europe has been greatly increased by the utter feebleness of the Labour government in giving up so much of Britain's rebate and the standing of the EU is at its lowest level for a long time in the UK, partly because the promised referendum has never been held."
