The "febrile" atmosphere around Brexit could be exploited by far-right extremists, the UK's most senior counter-terrorism officer has warned.
Assistant Commissioner Neil Basu said 18 terror plots were foiled in Britain since 2017, four of them far-right.
He said a "far-right drift into extreme right-wing terrorism" was a concern but officers were working to ensure groups did not gain a "foothold".
Mr Basu added leaving the EU with no deal would be "very bad" for policing.
The head of the Metropolitan Police's counter-terrorism operations was speaking at the launch of a new cinema advert aimed at encouraging people to report their suspicions about all forms of terrorism.
The 60-second film portrays a series of scenarios, including a man stockpiling hazardous material and another buying weapons.
Mr Basu revealed that a record 700 terror investigations are currently taking place, up from about 500 in March 2017.
Fourteen of the attacks foiled since 2017 involved Islamist suspects, Mr Basu said, adding that he was concerned about the possibility of radicalised fighters returning from abroad.
Another concern was the spread of propaganda online.
Mr Basu said while extreme right-wing activity was still a "relatively small threat", it was also "something we've got to pay very close attention to in this country - that we don't let that kind of far-right drift into extreme right-wing terrorism and we're working very hard to stop that".
Asked about the background of Brexit, Mr Basu told the BBC: "We saw a spike in hate crime after the referendum, that's never really receded.
"So there's always a possibility people are being radicalised by the kind of febrile atmosphere we've got at the moment.
"We want people to report anything that we think is going to lead to violent confrontation and people need to calm down and understand that we are paying very close attention to that and we will stop it wherever we see it."
Mr Basu said there was no intelligence pointing to an increased level of attacks after Brexit, but added: "What's most concerning me... is its potential to divide communities and set communities against each other."
His warning comes as Labour MP Melanie Onn revealed she had been threatened with being "gunned down".
The MP for Great Grimsby quoted the threatening email on Twitter, which was filled with swear words and called her a "traitor".
Ms Onn, who came out against another referendum on Brexit this week, said: "Everyone in Grimsby knows I've never backed down from a debate, even when I've had unpopular POV (including in referendum), but we must be allowed to have an opinion without this nonsense."
The threat against Ms Onn echoes the murder of Jo Cox, MP for Batley and Spen, who was shot and stabbed in 2016. Her killer, Thomas Mair, gave his name in court as "death to traitors, freedom for Britain".
It also follows concerns from a cross-party group of MPs that police were failing to prevent them being abused outside Parliament, with pro-EU Tory MP Anna Soubry being taunted with chants of "Nazi" during a live interview.
Mr Basu also told the BBC the possibility of a no-deal Brexit was "incredibly concerning" for police operations.
Echoing comments from Met Police Commissioner Cressida Dick in December, he said the UK and Europe would be in a "very bad place" if police could not exchange data or biometrics on suspected criminals and terrorists.
Mr Basu said the Met was working on contingency arrangements with police forces and agencies in Europe.
Home Secretary Sajid Javid said this week that he was confident that with or without a Brexit deal Britain would "continue to be a very safe country".