Police and intelligence services have disrupted 31 plots to attack Britain in the last four years, Ken McCallum, director general of the MI5 domestic intelligence agency, said on Friday.
The majority of plots were from Islamist extremists, but a growing number are organised far-right groups, he said.
"Even during the pandemic period, we have all been enduring for most of the last two years, we have had to disrupt six late-stage attack plots," McCallum told the BBC.
"So the terrorist threat to the UK, I am sorry to say, is a real and enduring thing."