Hundreds of British fighters who joined ISIL are believed to be hiding in Turkey, according to a report from The Times.
The information has prompted fears of an increased risk of terror attacks in Europe.
Earlier this year Andrew Parker, the director general of MI5, said the UK had seen “a dramatic upshift in the threat” from Islamist terrorism this year, reflecting attacks that have taken place in Westminster, Manchester and London Bridge.
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He said Britain is facing its most severe ever terrorist threat and fresh attacks in the country are inevitable.
As ISIL have lost ground in Raqqa and Mosul, it is thought that at least 300 Britons fled as the extremists fled.
About 850 Britons travelled to join Isis, of whom about half have returned. About 130 are confirmed dead.
The Times reports that British security sources warned yesterday that there was a risk that the extremists could try to return to the UK and may intend to carry out attacks.