The UK is likely to face an Islamist terrorist threat for the next 30 years, the former head of MI5 has warned.

Jonathan Evans, who stepped down as director general of the spy agency in 2013, described the threat as a “generational problem” which will take decades to tackle.

Lord Evans also claimed the Westminster Bridge attack earlier this year may have had an energising effect on extremists.

Meanwhile, he said he would be “very surprised” if Russia had not tried to interfere in the UK's last general election.

Lord Evans, who now sits in the House of Lords as a crossbench peer, told BBC Radio 4’s Today programme: “I think on the terrorism side we are at least 20 years into this. My guess is that we will still be dealing with the long tail in another 20 years time.