Britain's head of counterterrorism said a no-deal Brexit would create an "immediate risk" to the security of the country because of British police losing access to European data on serious criminals.
In an interview in The Guardian newspaper, Scotland Yard Assistant Commissioner Neil Basu said he "can't put a scale" on the damage leaving the European Union at the end of October without alternative measures in place would do to the country's security. No amount of preparation would lessen the impact, he added.
"It would create an immediate risk that people could come to this country who were serious offenders, either wanted or still serial and serious offenders committing crimes in this country, and we would not know about it," Basu said.
"It creates that risk. There would still be deep concern. There would be some damage to our safety. I can't put a scale on that."
A no-deal Brexit would mean British security services would lose access to data stored in the Schengen Information System, records of passenger names, and the ability to use European arrest warrants.
On the matter of security, Basu said any preparatory steps would result in less efficient systems. "Those systems and tools were developed in the EU for very good reason. They were very good," he said. "In a no deal we'd lose all that. We'd have to renegotiate it."
British Prime Minister Boris Johnson insists the already twice-delayed Brexit process will happen at the end of October, without a deal if necessary.
Johnson says the country must prepare for a no-deal exit and its consequences. The chief operating officer of Britain's Food and Drink Federation, Tim Rycroft, told the BBC that no-deal would result in "selective shortages" of food for "weeks or months", and there is also widespread concern about how National Health Service medical supplies would be maintained.
Basu said he accepted that politics was not the business of police officers, who "shouldn't be the loudest in the room", because security was just one of many concerns that needed to be included in political decision-making. But he stressed that it was vital policymakers take into account the informed advice of police on the security implications of Brexit.
"Their first responsibility is the security of the nation. I think they absolutely get what needs to be done," he added.
