Will Brexit make it harder for the British authorities to track terrorists?

Many pro-Leave voices during the EU referendum claimed that Brexit would make Britain more secure, arguing that increased border controls would allow jihadis to be shut out.

In the wake of terror attacks on mainland Europe, including some carried out by Isis fighters who had returned to the continent using false passports and hidden among refugees, the official Vote Leave campaign claimed Brexit was the “safer option”.

But all four fatal terror attacks launched in London and Manchester the following year were carried out by British nationals, with only the London Bridge attack involving foreign jihadis, from Morocco and Italy.

Assistant commissioner Neil Basu, the head of UK counterterror policing, said security services had previously seen foreign fighters as the biggest threat, but “the threat was already here”.

Last month, he warned that the loss of intelligence and data sharing caused by a no-deal Brexit would make the UK less safe.

He raised particular concern about the Schengen Information System (SIS II), which provides instant data on criminals and terrorists, airline passenger name records and the European arrest warrant.

Britain will lose access immediately in the event of a no-deal Brexit, and the systems are guaranteed only until the end of a transition period if a deal is agreed.

The UK will also no longer be a full member of Europol, which houses the European Counter Terrorism Centre (ECTC).

The government wants a bespoke security treaty allowing the UK’s continued use of the tools, but Theresa May’s agreement was rejected by MPs and the prospect of no deal is moving ever closer.

Mr Basu said European security services will continue to share intelligence through bilateral relationships, but “to leave without being able to exchange data or biometrics on people who might be criminals or terrorists would be a very bad place for this country, and for Europe, to be”.

He also raised fears over Brexit’s “potential to divide communities and set them against each other” amid increasing far-right extremism.

In November, the security minister Ben Wallace acknowledged that “partnership working is the key to any successfully counterterrorism strategy”.

He warned that losing EU systems would deprive police and border officials of vital information, and make it harder to track and arrest suspects.

“I have seen intelligence from other EU countries that contribute to saving British lives and other countries interdict UK-bound terrorists,” he told a summit in London.

“Threats to our shared values that begin in Europe can quickly reach the shores of the UK, and a mere 20-mile stretch of water cannot protect the UK from 21st-century terrorism.”

Got an unanswered question about Brexit? Send it to editor@independent.co.uk and we’ll do our best to supply an answer in our Brexit Explained series