Police could be powerless to arrest foreign crime suspects on the spot if Britain loses access to European databases and agreements after Brexit.
Even when checks show an individual is wanted overseas, they could not be detained until a warrant is obtained from the courts, a senior officer warned.
He raised concerns that suspected offenders could disappear if forces have to rely on measures that are slower and more bureaucratic than the existing arrangements.
It was also revealed that police estimate the loss of EU tools will result in costs of around £20 million a year.
Deputy Assistant Commissioner Richard Martin, the national policing lead for Brexit, said that currently if an officer carries out a “name check” on someone they encounter, it is automatically run through both UK and European databases.
In the same scenario but without the EU tools, the officer would have to check a separate Interpol system.
Mr Martin said: “If that comes back with what they call a red notice, which is where somebody is wanted, we would then have to go to a magistrates’ court to get a warrant to go and arrest that person.
“We could not arrest that person in front of us.”
There have been warnings that checks on an EU national’s criminal history could take 10 times longer in a no-deal scenario, rising from an average of six days currently to 66.
Mr Martin suggested there is a “real risk” individuals could abscond.
“If they know they are likely to be incarcerated or charged, then I think that’s a very real possibility,” he said.
Under the terms of the agreement struck by Theresa May last year, both sides committed to establishing a “broad” and “deep” partnership across law enforcement, criminal justice and security.
But in a no-deal Brexit, UK agencies face being locked out of systems for exchanging data such as criminal records, alerts on wanted individuals, DNA, fingerprints and airline passenger information.
They will revert to alternative conventions, international policing tools and bilateral channels to enable extradition of suspects, trace missing people and share intelligence.
Mr Martin said: “There is a tool behind any that we might lose but it’s not a one for one capability.
“Every fall-back we have is more bureaucratic, it is slower.”
Asked if the UK would be less safe, he said: “Yes. I think there is a risk.
“Is the UK going to turn into criminal gangs running amok? No.
“I don’t think we will become a country of choice for crime.
“Policing is not going to stop.
“We are going to build up the capability as best we can but we will be much more limited than we currently are.”
He said criminals are “entrepreneurs of crime” who could attempt to exploit any gaps.
The International Crime Coordination Centre has calculated that the annual cost to policing of losing EU law enforcement tools will be £20 million.
Mr Martin said there could be an impact on the work of frontline officers.
“If something takes two or three times as long as before, that’s probably another couple of hours where you are not back on the streets,” he said.
In a no-deal departure, British authorities would lose access to tools including the Second Generation Schengen Information System (SIS II), a vast database of alerts which was searched by UK officers more than 500 million times in 2017, and the European Criminal Record Information System, which is used to exchange tens of thousands of pieces of information about criminal convictions.
Extradition requests are also expected to take longer if Britain has to rely on a 1957 convention in place of the European Arrest Warrant.
Policing Minister Nick Hurd said the agreement reached with the EU would provide for an implementation period during which the UK would continue to use all the security tools and data platforms as now, while contingency plans for a no-deal scenario involve moving to “tried and tested” alternative mechanisms.
“Whilst we have been clear that these would not be like-for-like replacements, they are already used for police and judicial co-operation with many non-EU countries,” he said.
“Our primary objective, however, remains to secure a deal that protects mutually beneficial capabilities for both the UK and the EU member states.
“The continued safety and security of both UK and EU citizens is of paramount importance.”
Shadow home secretary Diane Abbott accused the Tories of showing “nothing but contempt for our safety and security”, warning: “Losing our real-time access to EU security tools and measures will be catastrophic for our security.”