Assistant Commissioner Neil Basu said Wednesday that the security threat would increase if Britain is not able to exchange data or biometrics on suspected criminals and terrorists as it currently does with EU nations.
Britain is scheduled to leave the EU bloc on March 29. A "no-deal" Brexit is possible because the British Parliament has rejected the arrangement the government negotiated with EU leaders.
Basu said counter-terrorism police stationed in various embassies and agencies throughout Europe are working on contingency plans to handle a "no-deal" departure.
"They are working very hard with their counterparts to understand how we cope if there is a no-deal Brexit, but as I said, a no-deal Brexit for operational security would be a very bad thing," he said.
Police are also concerned that it will become harder to bring suspects and fugitives back to Britain from other EU countries once Britain leaves the bloc and the streamlined European Arrest Warrant system.
Suspects being held in EU countries and sought by Britain will be able to slow the process considerably by taking advantage of a series of appeals processes that are currently unavailable, police say.
Basu said Britain has experienced a rise in hate crime, a rise in far-right rhetoric and a rise in far-right groups since the 2016 Brexit referendum. He said however that the growth has not been "explosive."