Neil Basu is also concerned at possible inability to share intelligence with EU partners

Britain’s counter-terrorism chief has said he fears the far right will exploit Brexit tensions with their propaganda triggering rises in hate crime and creating an atmosphere that terrorists can exploit.

The Metropolitan police assistant commissioner Neil Basu, head of counter-terrorism policing, told the Guardian he was concerned that far rightwing rhetoric from those lawfully allowed to operate will fuel tensions that spill over, in the same way Islamist propaganda incites terrorism.

He also said a no-deal Brexit leading to a loss of intelligence and data-sharing with Europe would leave its 27 nations and Britain less safe and was of “deep concern”.

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Basu, who briefs the prime minister and home secretary regularly, said police were working to minimise the damage but operations would be slower and more cumbersome if Britain left without a deal – and ministers knew this.

Every part of policing is struggling to deal with the different Brexit scenarios, from no deal through to no Brexit, as Britain nears the 29 March deadline to leave the EU mired in uncertainty.

While that is in the balance, the mounting divisions around Brexit led the senior police chief to voice his concerns about the effect in Britain.

Basu said: “My concern is the polarisation, and I fear the far-right politicking and rhetoric leads to a rise in hate crime and a rise in disorder.”

He said he was seeing an increase in far right activity from small but vocal groups as the Brexit saga continues, adding: “I am concerned about a small number of individuals trying to make a name for themselves such as Tommy Robinson.”

Basu drew a distinction between the far right whose rhetoric and activity is lawful, and the extreme right wing, which includes proscribed terror groups such as National Action.

Basu, who is also Britain’s most senior minority ethnic police officer, said far right rhetoric may not be illegal, but it fuels tensions and he fears its potential effects.

“It generates a permissive atmosphere to people who want to take their argument to more extreme levels,” he said. “There is a difference between being offensive and criminally offensive behaviour, and that is the line we have to monitor.”

Asked for examples, Basu said: “The melodramatic claim that this country is being overtaken by sharia law, that kind of far right discourse, that Asians are only interested in grooming children.”

While debate about such issues is lawful, the exploitation for extremist purposes concerns Basu because it could push susceptible people towards more extreme behaviour “in the same way extremist Islamist rhetoric radicalises people”.

Basu said indicators used by police to measure how communities are feeling were already showing some rise in tensions. While there was no intelligence of public order threats, police were planning for them, he said.

“We are monitoring if the intelligence suggests public order difficulties because of Brexit. So far it is not. We are planning for it, because it is sensible to plan for it. That potential exists.”

Basu is well regarded in Whitehall and tipped as a future commissioner of Britain’s biggest force.

He was speaking to promote a new advert in cinemas urging people to report suspicious terrorist activity as figures show 18 plots have been thwarted since March 2017, with 14 inspired by Islamist extremism and four from the extreme far right.

Basu’s counter-terrorism network remains under pressure, with Isis’s loss of lands in Iraq and Syria not leading to a decrease in work for police and the security services, with a record 700 live investigations ongoing.

Against that backdrop he spoke out about the damage no deal would cause, meaning a loss of key intelligence and data-sharing measures, with replacements being slower.

The three key tools that he said were of “deep concern” to counter-terrorism investigators were fast-time access to intelligence and data through the SIS II [Schengen information system 2] database, as well as passenger name records, and the ability to use European arrest warrants.

Basu said the damage from a no-deal Brexit to policing and security could be serious. “It is a very serious flaw in our security arrangements,” he said. “If we have no-deal Brexit, and we could not share that information, and if we lose access to those systems, it will inevitably make the UK and Europe less safe than it is today.”

Basu added: “There will be gaps, and it will be slower and clunkier. We have set up systems to try to deal with a no-deal Brexit. But it will be nowhere near as good as what we have got today.”

He said his European counterparts were keen to continue the relationship, and parts of cross-border police and security service counter-terrorism cooperation were covered by arrangements independent of the EU.

Asked in terms of policing and security if a deal was better than no deal, Basu said: “Yes. A deal leaves us in the same position as we are today or better.” He also said the security and policing arrangements under the prime minister’s deal with Europe, rejected last week by MPs, would have mitigated the damage.

Earlier this month Brexit tensions spilled over as remainer MP Anna Soubry was surrounded by pro-Brexit campaigners, who some claim were extremists, amid allegations her treatment was criminal and that police did nothing.

Basu said the protests had been peaceful for months before, but accepted police should have done more, adding: “I have accepted we did not act as we should have done. Anyone being harassed or intimidated as they go about their lawful business, we should intervene. It will not happen unchallenged again.”

The assistant commissioner said he had been into parliament to tell the Speaker and MPs this.