ISLAMIST terrorism will remain a threat for another 20 or 30 years, a former head of MI5 has said.

Jonathan Evans said the terrorist threat to Britain was a “generational problem”, and claimed 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 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 – I think this is genuinely a generational problem.

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“I think we are going to be facing 20 or 30 years of terrorist threats and therefore we need critically to persevere.”

He said the London suicide bombings in July 2005, which killed 56 people including the four bombers, triggered an “energising effect on the extremist networks in the UK”, and thought there would be a similar feeling following the Westminster Bridge attack this year. Since the atrocity in March, there have been attacks in Manchester, London Bridge and Finsbury Park.

Lord Evans, now an independent crossbencher in the House of Lords, also said 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, because 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.”

Lord Evans said he did not think security services were “losing the race” despite the spate of terrorist attacks in Britain this year.

“I don’t think we are losing the race; I think it is a race and we have to keep moving forward rapidly unless we are going to slip back. There will be some capabilities that do get eroded and I think the widespread use of encryption has reduced the ability of the agencies and the police to access the content of materials.”

He also said he was “rather surprised” terrorists had not used a dirty bomb in an attack on the UK.

“It seems such a clear opportunity. The fact we haven’t seen that is, of course, very, very good news.”