The Conservative government left the UK wide open to the far-right violence erupting across parts of the country by ignoring red flags and stoking fires with a culture war agenda, a senior adviser on extremism to Tory prime ministers has said.

Dame Sara Khan, who was Rishi Sunak’s independent adviser for social cohesion and resilience until May this year and acted as counter-extremism commissioner under Theresa May and Boris Johnson, said the recent administrations had failed the British people.

Repeated and urgent counsel that far-right extremists were exploiting gaps in the law to foment violence on social media had been ignored while top-rank politicians in a series of administrations sought to gain advantage by waging culture wars, Khan said, in a damning intervention.

“The writing was clearly on the wall for some time,” Khan said. “All my reports have shown, in a nutshell that, firstly, these extremist and cohesion threats are worsening; secondly, that our country is woefully unprepared. We’ve got a gap in our legislation which is allowing these extremists to operate with impunity.