The UK will be less safe after Brexit unless "mission critical" EU security arrangements are retained or adequately replaced, a Lords report warns.
Peers emphasised the importance to Britain's law enforcement agencies of a string of tools which have fallen under the spotlight since the referendum in June.
Senior figures in policing and counter-terrorism have highlighted the role played in the work by the European Arrest Warrant, the Second Generation Schengen Information System (SIS II) - a database of real time alerts, the European Criminal Records Information System (ECRIS) and Europol, the EU's law enforcement agency.
A report from the Lords EU Home Affairs Sub-Committee said: "The arrangements currently in place to facilitate police and security cooperation between the United Kingdom and other members of the European Union are mission-critical for the UK's law enforcement agencies."
The committee said that evidence it heard over the course of the inquiry pointed to a "real risk" that any new arrangements put in place by way of replacement when the UK left the EU "will be sub-optimal relative to present arrangements, leaving the people of the United Kingdom less safe".
Access to EU law enforcement databases and data-sharing platforms was "integral" to day to day policing up and down the country, according to the assessment.
It warned that if the UK lost access to them upon leaving the bloc, information that could currently be sourced in seconds or hours could take days or weeks to retrieve.
This would deliver an "abrupt shock" to UK policing and pose a risk to the safety of the public, the report said.
The UK and other EU member states shared a "strong mutual interest" in ensuring there was no reduction in the level of safety and security afforded to their citizens post-Brexit, according to the paper.
But the peers cautioned against "assuming that because there is a shared interest in a positive outcome, negotiations will unfold smoothly".
Baroness Prashar, chair of the committee, said: "Protecting the lives of its citizens is the first duty of government and should be the overriding consideration during Brexit negotiations.
"Without access to these vital EU tools or credible substitutes, we would be seriously harming the capability of our law enforcement agencies to fight crime and keep the public safe.
"Considering how instrumental the UK has been in shaping EU cooperation on police and security matters we hope the EU acknowledges the vital contribution we have and can continue to make."
Policing Minister Brandon Lewis said: "The UK is leaving the EU, but co-operation on law enforcement and security with our European and global allies remains a priority for the Government.
"We will do what is necessary to keep people safe and we are working, alongside policing and security partners, to explore options for co-operation arrangements once the UK has left the EU."