Government plans to opt out of 130 European Union police and criminal justice measures could weaken the UK's ability to fight crime, peers say.
The House of Lords EU committee said ministers failed to make "a convincing case" for repatriating the powers.
Home Secretary Theresa May says some of the joint measures are defunct .
Opting out would include leaving the European Arrest Warrant, which is used to speed up the extradition of criminal suspects between member states.
Ministers must decide by the end of May 2014 whether the UK should completely accept or reject 130 joint arrangements.
Ministers say they want to opt out of the package because the UK does not need to be bound by them - but then negotiate to rejoin individual measures where it is in the national interest to do so.
The most important and powerful of the measures is the European Arrest Warrant.
Other measures that could be ditched include arrangements to speed up sharing suspects' DNA profiles and fingerprints and joint working in specific areas such as terrorism, human trafficking or football hooliganism.
The peers said that while the UK could theoretically make alternative arrangements with EU states, they would be legally more complicated, expensive and less effective, thereby weakening the hand of British police.
The committee said: "In light of the evidence we have received, 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 in the UK, as well as reducing its influence over this area of EU policy."
Committee member Lord Hannay said: "Cross-border co-operation on policing and criminal justice matters is an essential element in tackling security threats such as terrorism and organised crime in the 21st Century and we need to ensure that the UK police and law enforcement agencies continue to have the tools they need to increase these increasing threats."
Mrs May told MPs last October that the government did not need to be bound by the measures because while some were useful, others were entirely defunct.
The committee said they had asked ministers for a list of the defunct measures, but so far had only been given three.
A Home Office spokesman said: "Discussions about which measures we may seek to opt back in to are ongoing but we have made it very clear that any decision will be guided by what is in our national interest.
"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."
But shadow home secretary Yvette Cooper accused Theresa May of a "shameful dereliction" of her duty in an attempt to appease Conservative Eurosceptics.
Ms Cooper said: "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 co-operate with forces abroad."
Liberal Democrat cabinet minister Danny Alexander said: "I am are clear that any final package will have to ensure the UK's continued participation in all the key measures which are important for public safety."
He said these included the European Arrest Warrant and Europol, the EU's law enforcement agency.