The UK may continue to face the threat of Islamist terrorism for another "20 to 30 years", the former head of MI5 has said.
Lord Evans told BBC Radio 4's Today programme the issue was a "generational problem" and that the UK needed to "persevere" with efforts to defeat it.
He also said devices connected to the internet needed to be made more secure in the face of emerging cyber threats.
And he warned that Russia was likely to try to interfere in the UK's democracy.
Lord Evans stepped down as the director general of the security service MI5 in 2013 at a time when it seemed that the terrorist threat from al-Qaeda might be subsiding.
But now, with the rise of the so-called Islamic State militant group, he said the threat was unlikely to end soon.
Lord Evans said: "There's no doubt that we are still facing a severe terrorist threat but I think its also important to put this in a slightly longer context because of 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."
He added: "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."
Lord Evans also warned that the "internet of things" - in which vehicles and domestic devices are all connected to the internet - will need to be made more secure to reduce emerging cyber threats.
He said it was concerning that Russia had apparently been interfering in the democratic processes of some Western countries.
Lord Evans said it was something the UK needed to be alert to, as he would be "very surprised" if it was not a target as well.
