The Home Office has provided £2 million of funding and around 50 police officers and staff will spend the next six months working in the contingency plans.

But Ms Thornton warned that the new arrangements would inevitably be slower, more bureaucratic and less effective and a no-deal Brexit would make it harder to keep people safe.

She said: “A very current example would be the two GRU officers who were responsible for the Novichok attack. If they are to step out of Russia into Europe there are European Arrest Warrants waiting.

“If we lose the European Arrest Warrant on the 29 March because there is no deal and no agreement then we have to rely on the 1957 [extradition] convention and it is longer and more complicated extradition processes and they don’t always apply in all countries either.

“So the whole point of what we are doing over the next six months is really understanding what these fall backs, how they can work, how we can get that information to detectives investigating in forces so as far as possible we can mitigate the loss of those tools and those capabilities.”