Islamist terrorism will continue to be a threat facing Britain for another 20 to 30 years, according to a former head of MI5.
Jonathan Evans said the terror threat to the UK was a “generational problem”.
He also said the Westminster Bridge attack earlier this year may have had an energising effect on extremists.
Lord Evans, who retired from the security service in 2013, told BBC Radio 4's Today programme: "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.
"I think that we are going to be facing 20 or 30 years of terrorist threats and therefore we need absolutely critically to persevere."
He said the London bombings in July 2005 triggered an "energising effect on the extremist networks in the UK", and thought there would be a similar feeling following the Westminster Bridge attack earlier this year.
"We did see a huge upsurge in threat intelligence after July 7 and I suspect that there's the same sort of feeling in the period after the Westminster Bridge attack - that a lot of people who thought 'I'd like to do this' suddenly decided 'yep, if they can do it, then I can do it'."
Since the atrocity in March, there were attacks in Manchester, London Bridge and Finsbury Park.
Lord Evans, now an independent crossbencher in the House of Lords, also told the programme he would be "surprised" if Russia had not attempted to interfere with British democracy, following repeated allegations of Russian interference in foreign elections.
He said: "It would be extremely surprising if the Russians were interested in interfering in America and in France and in various other European countries but were not interested in interfering with the UK.
“Traditionally I think we have been seen as quite hawkish and therefore I would be surprised if there had not been attempts to interfere with the election."
