The head of Britain's MI5 intelligence service on Wednesday issued a warning over the "startling" pull of Islamic State (ISIS) propaganda among people who may have never even visited its Middle East bases.

"The pull of this propaganda is startling: of the plots thwarted by police and MI5 and our Western allies in 2018, 80 per cent were conducted by people inspired by the ideology of IS but who had never actually been in contact with it in Syria or Iraq," Parker writes in an article in the Evening Standard'.

The intelligence chief hailed the fall of the so-called caliphate of ISIS in Syria as a hugely symbolic and significant military defeat but warned against complacency.

"We also know that, despite their losses, IS' remaining members are intent on directing terrorist attacks around the world, including on European soil. And this ambition is shared by Al Qaeda, whose desire and capability to attack the West hasn't diminished while IS has been in the spotlight," he said, adding that the danger had also been illustrated by the suicide bombings in Sri Lanka.

Parker said MI5 and Britain's other intelligence agencies were trying to use artificial intelligence and machine learning to help them respond to the ISIS threat. But he warned that critical information needed to protect the public might stay out of reach because the haystack of online data worldwide was getting larger and harder to penetrate.

"The haystack is getting bigger and the needle smaller," he said.

As Director General of the MI5 Security Service, Parker is in charge of Britain's domestic counter-intelligence operations as well as coordination with wider intelligence networks of the country.

As well as the Islamist threat, he said some of the other challenges for MI5 included rising far-right activity and that posed by Irish republican dissidents "stuck in a time warp".

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)