Fears that Britain could turn into a "safe haven" for foreign criminals have intensified after peers spoke out against the government's plans to opt out of EU police and criminal justice measures.
A report from the Lords' European Union committee found the evidence suggested the Home Office's plan to opt out of 133 EU measures next spring will reduce Britain's influence in Europe and ruin cross-border cooperation on policing issues.
Peers backed Britain remaining in the controversial European arrest warrant (EAW). It is viewed in government as a "vital tool" against international crime but has been criticised for its disproportionate use and the lengthy pre-trial detention times faced abroad by British citizens.
"If the United Kingdom were to leave the EAW and rely upon alternative extradition arrangements... it is inevitable that the extradition process would become more protracted and cumbersome, potentially undermining public safety," the report stated.
It dismissed human rights criticisms of the EAW, pointed out the long periods of pre-trial detention in poor prison conditions could also occur under another extradition system.
Last October home secretary Theresa May told the Commons she was minded to opt out of all the measures under consideration.
"Operational experience shows that some of the pre-Lisbon measures are useful, some less so, and some are now, in fact, entirely defunct," she said.
Britain would be able to opt in to individual measures, but would have to get its moves rubber-stamped by parliament and the European Commission - making the process an agonisingly complex and drawn-out one.
Peers suggested the Home Office might have offered a different view if it had looked at the implications of the blanket opt-out more carefully.
Subcommittee chair Lord Bowness said: "While it would be theoretically possible for the UK to continue cooperating with other member states through alternative arrangements to the EU measures covered by the opt-out, we found that these would raise legal complications, and result in more cumbersome, expensive and less effective procedures."
This would lead to the UK's hand being "weakened", he warned.
"The most effective way for the UK to cooperate with other member states is to remain engaged in the existing EU measures in this area," Bowness said.
Thais Portilho-Shrimpton, director of Justice Across Borders said: "We hope the government listens to the recommendations from the House of Lords and reconsiders abandoning or putting at risk these vital crime-fighting measures.
"We urge the coalition and the opposition to show they are both tough on crime, care about the safety of British citizens, and that they will not turn the UK into a safe haven for foreign criminals."
Shadow home secretary Yvette Cooper accused the government of gambling with the nation's security in order to "appease the Conservative party's euroscepticism".
She said May was guilty of a "shameful dereliction of her office and duty to the public" and called on her to think again.
"At a time when cross-border crime is a growing problem and cross-border security threats remain significant it is completely irresponsible for Theresa May to be making it harder for the police to cooperate with forces abroad," she said.
The Home Office said discussions about which measures Britain may seek to opt back into are ongoing, suggesting the wholesale opt-out from pre-Lisbon treaty EU security measures remains the plan.
"We have made it very clear that any decision will be guided by what is in our national interest," a spokesperson added.
"We have made a commitment to a vote in both Houses of Parliament before we take a final decision to opt out. That vote will take place in good time before May 2014."