One in four Londoners have witnessed support for extremism in the past year, a survey has shown amid warnings that counter-radicalisation efforts are not reaching all communities.

A YouGov poll for the Mayor’s Office for Policing and Crime showed 25 per cent had witnessed views “promoting, endorsing or supporting extremism”.

The research suggests that as many as 1.5 million London adults may have been exposed to hate speech, in the wake of four terror attacks by Isis supporters and a far-right terrorist in the capital.

“It’s clear that our ability as a country to tackle radicalisation and violent extremism is simply not good enough, and this is putting our safety and security at risk,” mayor Sadiq Khan said.

“I don’t think there are things such as hard to reach communities, there is hard to reach government and we need to do a far better job at making sure we engage with all Londoners.”

Speaking to terror attack survivors, former extremists, religious leaders and counter-radicalisation campaigners gathered at London’s City Hall, Mr Khan said authorities needed to gain the trust of people exposed to extremism of all kinds.

He called for a “full and frank assessment” of existing government programmes to tackle low public engagement, adding: “Too many communities now feel distrustful and alienated by Prevent and this is a fundamental flaw that needs to be addressed.”

Research by the mayor’s office, commissioned before the government announced a review of its flagship counter-extremism programme last week, concluded that it needed to be overhauled.

The survey also showed that almost two-thirds of Londoners would not know how to seek support over terror concerns, and less than a quarter of respondents said they could spot the signs of extremism.

Mr Khan made an appeal for renewed efforts to promote cohesion and tolerance, amid heated divisions over Brexit.

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“Every single day, the real and constant threat from violent extremism weighs heavily on all our minds,” he added.

“The best way we can honour those we lost is not only to ensure we uphold and cherish the values the extremists most hate – democracy, justice, equality and openness to others – but to get much better at rooting out those who seek to spread these vile ideologies.”

Mr Khan told The Independent that although the past year has seen no terror attacks in Britain, 18 plots had been foiled and there was an increased threat from both Islamists and far-right extremists.

“What you need to do is stop people getting in the pipeline to become a terrorist, and that means countering violent extremism,” he added. “The threat is still there.”

The mayor said he had spoken to both white working-class Londoners and those from Muslim, immigrant backgrounds who felt neglected by the state.

“If you look at the radicalisation paths of far-right extremists, they’re very similar to the radicalisation paths of Islamist groups,” he added.

“People not having a sense of belonging, being worried about the other, having a sense of grievance. And then somebody comes along, usually somebody who is charismatic, who diverts you away to the often nihilistic views that they’ve got as the solution … people are being radicalised and groomed in their bedrooms via the internet.”

Terror police appeal for public help to thwart plots

Mr Khan said he was concerned that Brexit would cause a spike in hate crime mirroring that following the 2016 EU referendum, and said police were contingency planning for potential civil unrest.

He has personally been targeted by right-wing activists and anti-Islam groups including the UK “yellow vests”, who hold him responsible for rising crime and other grievances.

Extremists frequently post doctored images of Mr Khan and racist memes on social media, and critics crowdfunded a balloon depicting the London mayor in a bikini to emulate an anti-Donald Trump protest.

The Finsbury Park attacker, Darren Osborne, named Mr Khan in a suicide note written before he ploughed a van into Muslims.