The Deputy Prime Minister Nick Clegg has warned that Britain could become a "safe haven" for foreign criminals if it pulls out of the European Arrest Warrant.
The Conservative Home Secretary Theresa May plans to opt out of 133 EU-wide justice and policing measures, including the EAW.
After meeting campaigners, Mr Clegg said:
While some measures of European co-operation on crime are old, out of date or defunct, the police and other law enforcement agencies consistently tell us that other measures are essential for our national security and public safety.
The European Arrest Warrant is one of those key measures...as the police say, without it Britain could become a safe haven for Europe's criminals.
We want to improve the way the arrest warrant works.
But this key crime fighting tool should be reformed, not abandoned.
One of Mr Clegg's aides went further, telling the Financial Times:
It is incredible that people would risk the security and safety of British citizens for some anti-European posturing.