A no-deal in Brexit trade talks at the end of the year would slow down the police and make it harder for them to catch criminals from European countries, MPs have been told.
Richard Martin, the Metropolitan Police's lead for Brexit, said the contingencies in place to deal with disruption if a trade deal is not reached were "not as quick, or as effective" as the systems currently in place.
The deputy assistant commissioner told the Commons EU future relationship select committee on Tuesday that "instant, at your fingertips" access to EU-wide criminal databases would be lost to police on the ground.
Police and judicial cooperation is one of the main stumbling blocks in EU negotiations, with matters complicated by a UK refusal to give written assurances Britain will remain a party to the European Convention on Human Rights that guarantees the rights of European citizens will be respected.
"It’s at your fingertips, it’s live time, it’s immediate. If we lose that capability, we will revert to Interpol notices which are slightly different They don’t have the full capability of SIS2 [a database],” Mr Martin told the committee.
“Our use of the European arrest warrant combined with SIS2 [a database] means that my officer, who may be in uniform stopping a car on the streets of London, can identify somebody immediately if they are may be wanted for murder in France. Or one of our other member states can immediately arrest them and put them through the extradition process.
“The worst-case scenario is my officer is left in an ambiguous position where [if] they do stop a car, they do stop somebody that’s wanted for a very serious offence. And if there are no other grounds to arrest that person there and then, they’ve got to run off to Westminster magistrates court, which is the only court in the land that can issue the warrant. So, yes, there’s a big risk that they could have abscond or disappear."
The warning comes a day after the Home Secretary said foreign criminals would be reported under the UK's new post-Brexit border operating model.
The government has said it wants to reach a deal with the EU by the autumn but that it would be prepared to leave the single market and customs union at the end of the year without an agreement.
While the government refers to the arrangement as an "Australia style Brexit", the approach would in fact represent a significant upheaval of UK-EU relations overnight and is expected to cause major disruption.
"Losing access to viral security tools would represent a huge failure on the part of UK negotiators," said Naomi Smith, chief of the pro-EU campaigning organisation Best for Britain.
"As the UK police lead on international criminality says, ending the transition period without a good deal on security would severely damage Britain's ability to catch criminals who flee.
"The UK has made over a thousand arrests in the last decade using the European Arrest Warrant, while Interpol and the European Criminal Record Data Exchange have also been critical.
"Without these invaluable tools, Brits will be more vulnerable in 2021 and beyond."