The UK’s safety and security would suffer from a no-deal Brexit and no amount of planning and preparation can erase the risk, Britain’s head of counter-terrorism has said.

The Scotland Yard assistant commissioner, Neil Basu, said key crime-fighting tools would be lost and their replacements would not be as good.

Speaking in a wide-ranging interview in which he also warned that boosts to police and security service numbers were no longer enough to combat terrorism, he said: “We can make them [the damaging effects] less, but they would be slower systems. Those systems and tools were developed in the EU for very good reason. They were very good. We had just signed up to biometric sharing.

“In a no deal we’d lose all that. We’d have to renegotiate it.”

The three key measures are fast access to intelligence and data through the Schengen Information System II database, as well as passenger name records, and the ability to use European arrest warrants.

Basu said: “We have done a lot of contingency planning to put things in place. But there are some things you can’t put in place. So there is no contingency planning for not being given passenger name records.

“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. It creates that risk.

“With my police leadership hat on there would still be deep concern. There would be some damage to our safety. I can’t put a scale on that.”

Basu suggested that any plausible change would be for the worse. “If I willed it I would want the negotiation to produce the same agreement, so we had access to all the same tools, and the same data-sharing capability … that would be the best position.”

He accepted that decisions about how Britain’s EU departure was carried out were for politicians and that the voice of senior police officers “shouldn’t be the loudest in the room”, meaning security concerns were only one of many factors for politicians to consider.

Basu said politicians had listened to and understood police advice on the operational effect of Brexit. “Their first responsibility is the security of the nation. I think they absolutely get what needs to be done.”

A no-deal Brexit would mean the return of Interpol alerts and the UK relying on the 1957 European convention on extradition, under which it could take years to return a suspect instead of the current six weeks with the European arrest warrant. “There are lots of things we can do but they are no way near as good the systems that were developed and we would want those good systems to remain,” Basu said.

Amid controversy about comments by Boris Johnson such as saying women who wore the burqa resembled “letterboxes” and using the racially offensive term “piccaninnies”, Basu said: “Every public figure who’s got a microphone and has got an opportunity to speak should take the opportunity to be bringing society together.

“The most important thing everybody should be aiming for is a socially cohesive, inclusive society.”

Asked if he would allow someone to join the police if they had used such langage, Basu said: “No, they wouldn’t be recruited into policing.”

He added: “I’m not going to comment on individual comments by individual public servants. I’ve made the point that we should all be very careful about what we say publicly.

“As an organisation we have prided ourselves for a very long time … of being an organisation that wants to be anti-racist, inclusive, wants to be diverse.

“What we must not do at any point, now or in the future, is lower our standards in terms of integrity and professionalism, diversity, inclusion.”