Opting out of European police and justice measures could have "significant negative repercussions" for Britain's security and ministers have failed to make a convincing case for doing so, peers have warned.
A report by the House of Lords EU committee said it was particularly concerned about the impact of the UK withdrawing from the European Arrest Warrant (EAW) - insisting the alternatives were inadequate.
And it said the Government had failed to consult properly on the issue. Under the 2007 Lisbon Treaty, the Government has until May 2014 to exercise an opt-out from a package of 133 measures including the EAW.
Home Secretary Theresa May has indicated that the Government plans to repatriate the powers - then negotiate fresh deals on some elements. She has declined to confirm which measures she will seek to sign back up to amid fraught negotiations with Liberal Democrat coalition partners.
Deputy Prime Minister Nick Clegg has warned Britain could become a "safe haven'" for foreign criminals if it pulls out of the warrant system. The committee concluded that the arguments of those pushing for withdrawal were "not supported by the evidence we received".
It said it had "failed to identify any significant, objective justification" for avoiding the measures coming under the jurisdiction of the Court of Justice of the European Union - which will happen if the opt-out is declined.
And while it was "theoretically possible" for the UK to set up alternative co-operation arrangements, they would "raise legal complications and result in more cumbersome, expensive and less efficient procedures".
Renegotiating would be "time consuming and uncertain", the peers warned, and could leave "gaps and legal uncertainties" during any transition. Alternatives to the EAW would be as prone to the "serious injustices" which had occurred in cases involving the existing system, they said, arguing reform would be better achieved through negotiation with member states and the use of existing provisions.
"Relying upon alternative extradition arrangements is highly unlikely to address the criticisms directed at the EAW and would inevitably render the extradition process more protracted and cumbersome, potentially undermining public safety," the committee concluded.
"In light of the evidence we have received, including a preponderant view among our witnesses from the legal, law enforcement and prosecutorial professions, we conclude that the Government have not made a convincing case for exercising the opt-out and that opting out would have significant adverse negative repercussions for the internal security of the UK and the administration of criminal justice."