BRITAIN'S security would be adversely affected in a significant way if the UK Government went ahead with opting out of a raft of European police and justice measures, a highly critical House of Lords report has said.

The second chamber's European Union Committee argued that the Coalition had failed to make a convincing case for opting out of the measures, and made clear it was particularly concerned about the impact of the UK withdrawing from the European Arrest Warrant (EAW), insisting the alternatives were inadequate.

Earlier this year, Scottish Chief Constables expressed their concerns about the prospect of the UK Government's plan, warning that Britain could become a "refuge for foreign criminals".

The Association of Chief Police Officers insisted opting out of the EAW and data-sharing measures would create more bureaucracy, cost money and remove officers from frontline policing.

The UK Government has to decide whether it will continue to be bound by 133 EU police and criminal justice measures by June 1.

If it decides to opt out, then the measures will cease to apply to the UK from December 2014.

Last October, Home Secretary Theresa May said the Coalition's current thinking was that the UK should opt out of all the pre-Lisbon Treaty measures and negotiate to rejoin individual ones if in the national interest.

Deputy Prime Minister Nick Clegg also warned against pulling out of the EAW system.

In their report, the peers emphasised how they were struck by the clear view of legal professionals who gave evidence to their inquiry about the "potentially negative implications" for the UK of opting out.

"On the basis of the evidence we have received we do not consider that the Government has made a convincing case for exercising the opt-out," said the report.