The Britain people should prepare for the threat from Islamist attacks to continue for another three decades, a former MI5 chief has said.

Lord Evans said the scale of the threat was “severe” and a “generational problem” that the UK needed to “persevere” to defeat.

Speaking on BBC Radio 4’s Today programme, he warned: “We’re at least 20 years into this. My guess is that we will still be dealing with the long tail in over 20 years’ time.

“I think this is genuinely a generational problem. I think we are going to be facing 20 to 30 years of terrorist threat and therefore we need, absolutely critically, to persevere.”

At the end of May, Government sources said there are at least 23,000 jihadists in Britain with 500 being subject to active investigations led by MI5.

Since 2013, there have been 19 attempted attacks on Britain, with a number successfully claiming lives, including Westminster, London Bridge, and Manchester.

"I think we are going to be facing 20 or 30 years of terrorist threats" – former MI5 boss Jonathan Evans #r4today pic.twitter.com/SDugSqiIGC — BBC Radio 4 Today (@BBCr4today) August 11, 2017

Lord Evans added: “There’s no doubt that we are still facing a severe terrorist threat but I think it’s also important to put this in a slightly longer context, because, right the way back from the 1990s, we have been experiencing difficulties from Islamist terrorists of one sort or another.

“Over that period, the threat has come and gone but the underlying threat has continued.

“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.”

23,000 Jihadists, the vast majority of whom MI5 and the Police don't have the time, money, or people to watch https://t.co/goUgrrIa73 — Breitbart London (@BreitbartLondon) May 27, 2017

In 2015, it was reported that the Muslim population of England and Wales had nearly doubled in 10 years, and a significant proportion is sympathetic to radical views. There is also a growing threat from terrorists trained abroad.

Twenty-seven per cent of British Muslims polled in 2015 said they had sympathy with the attacks on Charlie Hebdo – the French satirical magazine that published cartoons of the Muslim prophet Muhammed.

A further 78 per cent supported punishment for the publication of cartoons featuring Muhammed and 68 per cent backed the arrest and prosecution of British people who “insult Islam”.