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Home-grown extremists inspired by the “startling pull” of Islamic State propaganda online were behind four out of five terror plots in Britain and the West last year and continue to be the most potent threat the country faces, the head of MI5 warned today.

In his first solo article for a newspaper Andrew Parker said in the Standard that the danger from IS had not diminished despite its military defeat and the collapse of its so-called caliphate in Syria. This was because its extremist ideology did “not require territory to survive”, giving the terror group a continuing “ability to perpetuate misery through launching large-scale attacks”.

Eighty per cent of Western plots last year, including most in this country, involved extremists who had been inspired by IS without having any contact with its members in Syria or Iraq.

Mr Parker said the danger had also been illustrated by the murderous Easter suicide bombings in Sri Lanka and was being heightened because vital evidence needed to foil attacks was often “buried” online overseas.

As well as the Islamist threat, other challenges for MI5 highlighted today by Mr Parker included rising far-Right activity and that posed by Irish republican dissidents “stuck in a time warp”.

Mr Parker said MI5 and Britain’s other intelligence agencies were trying to use artificial intelligence and machine learning to help them respond to the IS 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.

He added: “Increasingly, the vital piece of information that might stop an attack is unlikely to be held by MI5 but buried somewhere else in the mountain of data generated each day, often scattered across the world.”

Mr Parker’s warnings follow the recent attention given to the threat posed by British foreign fighters overseas who might be returned to this country following the territorial defeat of IS in Syria.

More than 60 Britons, including dual nationals, are currently understood to be held in detention camps run by Kurdish forces awaiting decisions on where to send them.

They comprise around 30 women and a similar number of men, and include many regarded as battle-hardened and potentially highly dangerous. But in an attempt to shift the focus back onto the domestic Islamist threat, Mr Parker today made clear that the biggest danger was posed by home-grown extremists who have been inspired by IS propaganda, rather than directed from abroad.

Warning that “Islamist terrorism remains the most acute” of the “multiple terrorist threats facing the UK,” Mr Parker said the “fall of the so-called caliphate” had been a “hugely symbolic loss” for IS — which he referred to as Daesh — but did not mean its menace had ended.

“We must not be complacent. Although the collapse deprived them of the oxygen of physical space, Daesh has shown that an ideology does not require territory to survive,” he said.

“In the UK, there remain individuals who are inspired by Daesh propaganda despite having shown no interest in travelling to Syria. 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 Daesh, but who had never actually been in contact with Daesh figures in Syria or Iraq.

“We also know that, despite their losses, the intent of Daesh’s remaining members to direct terrorist attacks around the world, including on European soil, has not diminished. And this desire is shared by al-Qaeda, who still have the intent and capability to attack the West and have been patiently waiting while Daesh has been in the spotlight.”

Mr Parker said that his staff were doing “extraordinary things” to keep the country safe in the face of such dangers, thwarting plots and saving lives. But he cautioned that the increasingly “vast amounts” of material on smartphones and the scale of social media output meant that more data than ever was being created at the same time as encryption was making it harder for MI5 to access it.

He said this was making it easier for “those who wish to do us harm” and that terrorists were exploiting these trends to “radicalise … spread hate and fear and … plot murder”.

This meant that “simply being on our radar, or being under surveillance” was not always going to be enough to allow his officers to see the “critical” intelligence about potential terrorists. “The haystack is getting bigger and the needle smaller,” he added.

Mr Parker, who has been MI5’s director general since 2013, said that his spies were working with private firms, police and other public bodies to develop AI and machine learning technologies that could help to uncover key evidence more easily.

It was also “deepening” its ties with European intelligence agencies, with all sides committed to continuing this co-operation regardless of Brexit.

But he emphasised that public help remained vital too and appealed to anyone concerned about a potential threat to contact the anti-terrorist hotline on 0800 789 321.

Mr Parker’s remarks today follow the reappearance of IS leader Abu Bakr al-Baghdadi in a video released this week in which he made a renewed call for his followers to carry out attacks worldwide.