Withdrawal from the EU's law and order measures would leave paedophiles and criminals "running around freely in the streets", David Cameron has been warned.
Viviane Reding, the vice president of the European Commission who is responsible for justice, said the repatriation of powers on crime and policing would be "crazy".
The Prime Minister indicated earlier this year that the Government would opt the UK out of the laws, including the European Arrest Warrant, sparking a rift with his Liberal Democrat deputy Nick Clegg.
Britain's threat to opt out of certain EU legislation is a contentious issue
In an interview with the Daily Telegraph, Reding warned: "Do you want criminals and paedophiles running around freely on the streets? Is that really in the United Kingdom's interest? It is crazy."
Reding claimed that British police and law enforcement agencies were of the "same opinion" over any move to opt out of justice measures.
"It is the British police that made an outcry of horror when they heard the British Government wants to opt out of certain instruments that are essential for Britain to defend itself," she said.
Cameron is under pressure from Tory eurosceptics to loosen Britain's ties with the EU as the bloc's eurozone members move towards greater integration. He is expected to announce within weeks that he is prepared to hold a referendum on a new relationship between the UK and Brussels which he is determined to forge over the next two years.
However some, such as Ken Clarke, have said withdrawal would be "a disaster".
Reding indicated the strength of opposition he faces among EU bureaucrats, saying there was no possibility of Britain "repatriating" powers from the EU.
"You have to make up your mind, either you belong to it or you don't belong. There is no cherry picking," she said.
"That status quo cannot be undone. We will certainly advance to make it more coherent and stronger."
Cameron received a further warning from one of Britain's principal Maastricht Treaty negotiators from 1991 that he risks taking Britain out of the EU "by accident".
Lord Kerr of Kinlochard, who was the top civil servant at the Foreign Office between 1997 and 2002 said it was wrong to assume that other members of the EU would "leap to their feet" and accommodate the UK's demands at an intergovernmental conference (ICG) in 2015.
"I think that the Cameron strategy, if he was back in Downing Street with a majority, could lead to our leaving by accident. I think there is an analytical error," he told The Guardian.
"I think they genuinely believe people are going to leap to their feet to sing Ode to Joy or Rule Britannia and it is a done deal. And then we will have a referendum on this new deal.
"But supposing the rest of the EU don't leap to their feet? It doesn't take one lonely Czech. It takes them all. This is an IGC, so it is unanimity rules. They have all got to agree every word of the changes we want.
"You could find yourself in an awkward situation in which you are stuck with a referendum pledge on the new deal and there is no new deal, or there is a new deal so trivial that it is mocked by Ukip and the press. In either scenario, it seems to me there is a risk that Cameron finds himself arguing we must go. I am sure that it is not what he wants to do."
In comments that will embolden eurosceptic demands for British withdrawal from the EU altogether, former EC president Jacques Delors said that the UK could leave the union and have a "different kind of partnership" with Europe.
In an interview with German newspaper Handelsblatt, he suggested a looser arrangement based on economic free trade.
"The British are solely concerned about their economic interests, nothing else. They could be offered a different form of partnership," he said.
"If the British cannot support the trend towards more integration in Europe, we can nevertheless remain friends, but on a different basis. I could imagine a form such as a European economic area or a free-trade agreement."
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