Nick Clegg will attempt to dispel suggestions that he is forging closer links with David Cameron by placing the Tories' controversial policy on Europe at the heart of the Liberal Democrats' election campaign.
The Lib Dem leader will claim this week that the Conservatives threaten to turn Britain into a "safe-house for criminals" by planning to withdraw from European cross-border policing agencies.
Mr Clegg and Mr Cameron appeared together with Gurkhas' rights campaigner Joanna Lumley earlier this month, prompting suggestions that the two leaders could work together in a coalition government if the next election results in a hung parliament.
But the Tory leader's hard-line Eurosceptic position, including his plan to withdraw from the European People's Party (EPP) grouping of MEPs after the 4 June poll, is to be targeted by the Lib Dems when they launch their European election campaign on Tuesday.
Mr Clegg will say Mr Cameron's plans to create a new grouping, including a Polish party that opposes gay rights, is a sign of the Conservatives' desire to "disengage" from Europe.
The Lib Dems have seized on comments by MEP Timothy Kirkhope, Tory spokesman on European justice policy, that the UK should not be a member of Eurojust, the body that helps to co-ordinate prosecutions across Europe.
The Lib Dems also claim the Tories are becoming sceptical of Europol, the cross-border policing body.
On Thursday, the Lib Dem home affairs spokesman, Chris Huhne, will launch a dossier claiming that closer co-operation in the EU on drugs, child trafficking, hi-tech crime, terrorism and illegal immigration would be at risk if the Tories won power.
A senior Lib Dem source said: "David Cameron's approach to international crime is symptomatic of the Tory approach to all European issues. Isolationism comes before pragmatic co-operation, ideology before the safety of the public.
"The Tories would rather turn Britain into a safe-house for criminals than co-operate with other countries. What better evidence that David Cameron and his party are unfit to govern?"
But in an interview with The Independent on Sunday, the Tory leader denied he was disengaging from the EU as a result of his withdrawal from the EPP: "The fact is, we believe in being members of the European Union, we think that's good for Britain, we support free trade and co-operation between the nation states and the European Union, we are not anti-European co-operation.
"[But] we think Europe needs to change its agenda from being obsessed with institutional reform and endlessly taking further powers unto itself.
"I think actually forming a group of like-minded parties across Europe that share the elements of this agenda is a sensible thing to do."
He insisted that President Nicolas Sarkozy and Chancellor Angela Merkel "understand" his position and that the Conservative Party and its new grouping will be "good neighbours rather than unhappy tenants".
Mr Cameron added: "Of course, in any group, you may not agree with all your partners about everything. But the key thing is, on the issue of the shape and development of the European Union, are you basically in agreement?"
A source close to Mr Huhne said: "The Conservatives like to think of themselves as the 'tough on crime' party. But their failure to bring crime-rates down in the '80s and '90s exposed this as a myth. Crime doubled under their last tenure in government. Today, David Cameron is still struggling to articulate his approach to crime. Yet when it comes to cross-border crime, Cameron is sending out some clear signals. In the field of European police and judicial co-operation, the Conservatives are increasingly hostile. Indeed, it is difficult to see how a Conservative government so antagonistic towards Europe could continue the level of co-operation that exists today.
"The Conservative Party has reached the point where it now puts ideological opposition to Europe before catching dangerous criminals. Dogma now trumps pragmatic co-operation in the fight against crime."