Extremist terror will threaten the UK for decades, the former head of Britain's MI5 warned as the methods of radicalization and attack continue to adapt.
Jonathan Evans said Britain was facing a "generational problem" seeing extremists energized by recent attacks like Westminster.
"We're at least 20 years into this, my guess is that we will still be dealing with the long tail in another 20 years' time - I think this is genuinely a generational problem," he told BBC Radio 4's Today program.
"I think that we are going to be facing 20 or 30 years of terrorist threats and therefore we need absolutely critically to persevere."
Lord Evans, who now sits as a crossbencher in the House of Lords, said that when he retired from the domestic security service in 2013 he believed the UK was "over the worst of the al-Qaeda threat".
"But then we saw the development and emergence of Daesh [ISIS/ISIL] with the same ideology and many of the same people," he added.
The former MI5 director said the July 2005 London bombings had an "energizing effect on the extremist networks in the UK" that may have been repeated by the Westminster attack in March.
It was followed by Daesh-inspired massacres in Manchester and London Bridge, and then the far-right terror attack on Muslims in Finsbury Park - as well as six foiled plots.
"Since 2013 there have been 19 attempted attacks that have been disrupted and even since the attack at Westminster we are told there have been six disruptions, so this is a permanent state of preparedness," Lord Evans said.