The fall of Afghanistan to the Taliban is likely to have "emboldened" UK terrorists, the head of MI5 has warned.
Director general Ken McCallum told the BBC that while the terror threat would not change overnight, there could be a "morale boost" for extremists.
And there is also a concern terrorists could regroup and plan "well-developed, sophisticated plots", he said.
A total of 31 late-stage attack plots have been foiled in the UK in the past four years, said Mr McCallum.
That includes six during the pandemic period alone, he added. While they were largely Islamic extremist plots, there were also a "growing number" of attacks planned by extreme right-wing terrorists.
"The terrorist threat to the UK, I am sorry to say, is a real and enduring thing," he added.
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."
It is difficult to say if the UK is safer, or less safe, now than it was at the time of the September 11 attacks, said Mr McCallum.
So-called Islamic State had "managed to do something that al-Qaeda did not" in inspiring lots of people to attempt smaller scale acts of terrorism, he explained.
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."
https://www.bbc.com/news/uk-58512901
