More than half of MI5’s anti-terror operations involve Britons travelling to fight in Syria, Whitehall officials have revealed.
An estimated 400 people from the UK have gone to fight in Syria since the conflict there began, and 250 who have since returned are now being monitored by the security services.
At least one plot by a “returnee” is already understood to have been foiled – a “Mumbai–style” gun attack in central London.
Intelligence experts believe the conflict in the Middle Eastern country is now stoking the biggest terror threat to the West since the 2001 September 11 attacks on the United States.
The Syrian civil war – three years old this week – has become a magnet for radical Islamists around the world.
More than 2,000 fighters are estimated to have travelled to Syria from Europe, with many thousands more joining them from the Gulf states and north Africa.
UK police have made a spate of Syria-related arrests in recent weeks. Four people were taken into custody on Tuesday.
At the start of this month, Moazzam Begg, a former Guantanamo Bay detainee, appeared in court charged with funding terrorism in Syria, which he denies.
Efforts to stop Britons travelling to Syria to fight have included a blitz from the Home Office to counter online extremist content.
The government’s counter-terrorism internet referral unit is understood to have authorised 8,000 “takedowns” of content in the past eight weeks alone, compared with 21,000 similar legal removals over the past four years. In addition, the government is lobbying internet companies to better moderate content online.