Nick Clegg has warned that Britain could become a "safe haven" for foreign criminals if it pulls out of the European Arrest Warrant.
He issued the fresh warning shot to the Tories over plans to opt out of EU-wide justice and policing measures after meeting campaigners.
The EAW is one of a number of measures the government may exercise an opt-out
Among them was the mother of an ex-footballer badly beaten in Crete who complained "ideological battles in Westminster" were putting justice at risk.
Maggie Hughes believes four men found guilty by a Greek court of an attack which left son Robert with brain damage would have escaped justice if such a warrant had not been in force.
Home secretary Theresa May has indicated that the government plans to exercise its opt-out from 133 measures including the EAW - and negotiate fresh deals on some elements.
Under the 2007 Lisbon Treaty, the government must decide by May 2014 whether to withdraw or remain subject to the entire package.
Mrs May has declined to confirm which measures she will seek to sign back up to amid highly-divisive negotiations with Liberal Democrat coalition partners.
Mr Clegg signalled his determination to keep Britain within the arrest warrant system and other elements seen as "essential" by the police for national security and public safety.
"This was a very productive meeting which focused on an issue of huge importance - what do we need as a country to keep the public safe from crime, and deliver justice for the victims of crime," he said in a statement.
"While some measures of European co-operation on crime are old, out of date or defunct, the police and other law enforcement agencies consistently tell us that other measures are essential for our national security and public safety.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. Without it, victims of crime like Maggie may never have seen those who beat her son nearly to death brought to justice, and 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. That's why we're introducing some changes domestically, and why we're talking to our European partners about reform. But this key crime fighting tool should be reformed, not abandoned.
The deputy prime minister told Maggie Hughes that the EAW has to be protected
"Britain has a long and proud track record of leadership in the fight against organised crime and illegal immigration.
"I want to see us continuing to lead internationally on this agenda in to the future."
Mrs Hughes said: "We need to keep what we've already got, because without the European arrest warrant we may never have got the suspects back to Greece and justice for Robbie.
"My son's ability and other people's children's ability to get justice should not be threatened by ideological battles in Westminster.
"No-one has told me what the good reasons are to pull out of this cooperation, and Mr Clegg agreed we must protect the EAW and other important measures."
In a joint statement, Justice Across Borders, Anti-Slavery International, ECPAT UK and former Crime Watch presenter Nick Ross, who also attended the talks, said: "We were pleased the deputy prime minister signalled today his unequivocal support for the tough measures that are proven to be so effective in tackling international crime, such as the European arrest warrant.
"Victims and potential victims of human trafficking, and child trafficking would be increasingly vulnerable should the Government choose to abandon cooperation with our European partners and agencies, such as Europol.
"We must beware of those who are being soft on crime by pushing for this opt-out purely on ideological reasons."