A no-deal Brexit would damage police powers to detain foreign suspects and leave British fugitives in Europe beyond the law, a police chief has said.

Deputy assistant commissioner Richard Martin is leading national police preparations to cope with the loss of key crime-fighting measures if Britain leaves the EU on 29 March without a deal.

“Criminals are entrepreneurs of crime … if there is a gap to exploit I’m sure some of them probably would,” he said.

In the event of no deal, British police would lose access to a range of Europe-wide tools such as the SIS-2 database of convictions and wanted suspects, and the European arrest warrant, which speeds up extradition and allows arrests if officers suspect someone is wanted overseas.

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Martin said such a loss of powers would mean that police were unable to detain foreign suspects even if they were spotted in Britain, and even if Interpol had issued an international notice for their arrest.

“We could not arrest that person in front of us, while with an EAW we can do it instantaneously,” he said. “The officer has to go to a magistrates court to get a warrant under the 1957 convention of extradition,” he added.

Checks under the system could take up to 66 days. Asked if he was worried foreign criminals would abscond while UK police waited for information to come back, Martin said: “If we don’t get information back in a timely fashion … that is a very real possibility.”

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 scenario, UK agencies also face being locked out of systems for exchanging data such as criminal records, alerts on wanted individuals, DNA, fingerprints and airline passenger information.

They would have to revert to alternative conventions, international policing tools and bilateral channels to enable extraditions, trace missing people and share intelligence.

Martin said: “There is a tool behind any that we might lose but it’s not a one-for-one capability. Every fallback we have is more bureaucratic, it is slower … We go back to a slower, clunkier place.”

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Some of the pan-European tools that British officers could lose access to have been in use for more than a decade. They used the SIS-2 database 539 million times last year. “If we exit the EU without a deal that gets switched off overnight”, Martin said.

Around 17% of people taken into police custody areas in the UK are foreign nationals, rising to 27% in London, he said.

He also said it would be harder to detain and extradite suspects wanted in the UK such as Jamie Acourt, who was arrested in Spain on a drugs offence in Spain and returned quickly to Britain. He is also a suspect in the Stephen Lawrence murder.

Martin denied Britain could become a haven for criminals, but said the extra time and effort needed would damage already stretched frontline policing. “If something takes two or three times as long as when you were doing it before, that’s probably another couple of hours maybe you are not back on the streets … It will have an impact on the frontline.”

It would also impact the rest of the criminal justice system, with thousands of extra visits by police to magistrates for the warrants they would need.

Some forces need assistance to be fully ready for a no-deal Brexit, and are being helped by a new international crime coordination centre with 50 staff, with more being trained in the regions.

All this could leave Britain less safe in many ways, Martin said. Border staff, for example, would find it a lot harder to screen for past offences, “If you haven’t got access to some of those really critical systems like SIS-2, you probably won’t know what their convictions are,” he said.