Six terrorist plots in the last stages of planning were disrupted during the Covid-19 pandemic, the head of UK’s domestic security agency said on Friday.

Ken McCallum, the head of MI5, said that the six were among 31 disrupted in the past four years by police and the security services.

Most of them were planned by Islamist militants, but a growing number were from right-wing terrorists, Mr McCallum told the BBC.

“In the last four years, working with the police, my organisation has disrupted 31 late-stage attack plots in Great Britain,” he said.

The pandemic failed to halt the threat from extremists, with police saying that online radicalisers have sought to take advantage of the vulnerable spending more time in front of their computers during the pandemic.

“This is part of our lives at this time in history and I suspect some form of this terrorism will remain with us for quite some time to come,” Mr McCallum said.

On the eve of the 20th anniversary of the September 11 attacks in the US, Mr McCallum said it was too simplistic to say if Britain was now more or less safe from the threat of terrorism.

He said that on September 11, 2001, the UK was faced with a well-funded adversary in Al Qaeda with a safe haven and “we had to play catch-up in a dramatic way”.

“Five years after 9/11, we were still facing plots of that magnitude. We, with others, worked deeply and ferociously hard to reduce that style of risk from Al Qaeda.”

It has led to the emergence of smaller-scale acts of terrorism inspired by ISIS. “The number of plots that we disrupt nowadays are actually higher than the number of plots that were coming at us after 9/11, but on average they are smaller plots of lower sophistication,” he said.

But he said the Taliban takeover of Afghanistan may lead to a greater risk of terrorist attacks in the UK.

“There is no doubt that events in Afghanistan will have heartened and emboldened some of those extremists, and so being vigilant to precisely those kinds of risks [is what] my organisation is focused on, along with a range of other threats,” he said.

Tony Blair, who was prime minister at the time of the September 11 attacks, said that the overthrow of the Taliban would come from a revolt from within the country, but defended the decision to go to war in 2001.

“I don’t think if we’d left them alone after that or taken some counterterrorism measures – sent some special forces in, tried to take out the leadership – I don’t think the movement would then have died or gone away,” he told the programme.

“I think it is a genuine global, ideological movement.

“They believe it and they are going to pursue it and in the end will only be defeated, not because they give up and go away, but because they have both the ideological and security battle taken to them and in the end their own populations turn against them and defeat them.”