A total of 31 late-stage terror plots have been foiled in the UK in the past four years, the head of the UK's secret service, MI5 has revealed.
Director general Ken McCallum, who revealed in October there had been 27 attacks thwarted since 2017, said there had been six during the pandemic.
He said they were largely Islamic extremist plots, but a "growing number" if terrorist threats were planned by right-wing terrorists.
He also warned that the fall of Afghanistan to the Taliban will likely have "emboldened" UK terrorists.
The terror threat would not change overnight but there could be a "morale boost" for extremists, he said.
"The terrorist threat to the UK, I am sorry to say, is a real and enduring thing."
"Of course there are likely to be terrorist attacks on UK soil on my watch," he said, saying MI5 works as hard as it can to stop them happening but "to our horror, we know that won't be possible on every single occasion".
The head of the security service added that MI5 had "saved thousands of lives across the last 20 years" but it "cannot always succeed".
Mr McCallum, speaking to BBC Radio 4's Today programme on the eve of the 20th anniversary of the 9/11 attacks on the US, said that smaller-scale terrorist acts by those already in the UK made up the largest number of threats faced by MI5.
"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.
But there is still also a risk of an increase in larger plots directed by terrorism groups like al-Qaeda, he warned.
"The big concern flowing from Afghanistan alongside the immediate inspirational effect is the risk that terrorists reconstitute and once again pose us more in the way of well-developed, sophisticated plots of the sort that we faced in 9/11 and the years thereafter," Mr McCallum said.
While the government says it will judge the Taliban by their actions, the UK security service would plan for the possibility "more risk, progressively, may flow our way", Mr McCallum said.
"Terrorist threats tend not to change overnight in the sense of directed plotting or training camps or infrastructure - the sorts of things that al-Qaeda enjoyed in Afghanistan at the time of 9/11," he said.
"These things do inherently take time to build, and the 20-year effort to reduce the terrorist threat from Afghanistan has been largely successful.
"But what does happen overnight, even though those directed plots and centrally organised bits of terrorism take a bit longer to rebuild... overnight, you can have a psychological boost, a morale boost to extremists already here, or in other countries."
He added: "We need to be vigilant both for the increase in inspired terrorism which has become a real trend for us to deal with over the last five to 10 years, alongside the potential regrowth of al Qaeda-style directed plots."
The former chief of the UK defence staff, General Lord Richards said he believes they are closer to "another 9/11" following the US and UK's withdrawal from Afghanistan.
"I fear the Taliban and some extremist jihadist groups are, whatever they like to say, in each other's pockets," he told LBC. "Scores will be settled, debts will have to be repaid and there will be ungoverned space opened up in Afghanistan which those groups will exploit and the ability of the Taliban to actually manage them will be minimal."
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