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It is thought that up to 1,700 French jihadists have returned home from fighting in Syria and Iraq since 2012. Britain and Germany have both seen around 800, a major cause for concern for European intelligence agencies.

“A lot of French people are coming back,” said Didier le Bret, France’s intelligence co-ordinator. “They’ve got a feeling it’s not going that well.”

Several of the Paris attackers had trained in Syria and managed to return to Europe posing as refugees.

Until Turkey closed its border with Syria earlier this year, foreign fighters had been able to move in and out of its territory with relative ease. Crossing has become much more difficult, particularly if a fighter does not have permission from ISIL. If defectors do get to Turkey, they face rigorous interrogation from officials in both Turkey and their home country.

Foreign fighters, particularly Westerners, initially enjoyed a special status within ISIL and were paid more than Arab recruits. But jihadists have in the past few months seen their salaries cut and their privileges rescinded as they come under pressure from coalition air strikes and attacks on the ground.

Kyle Orton, a Middle East analyst at the Henry Jackson Society think tank, said he had seen a growing number of reports of dissent within ISIL.

“The disparity between the expectation and the reality of the caliphate has been a long-standing fact for the European ISIL fighters, and as things look bleaker it is likely some of them will be disillusioned enough to seek an exit,” he said.

“What’s troubling for Western security is the European ISIL fighters who make it out. It is extremely difficult to tell apart the genuine defector, the extremist who just doesn’t want to die, and the ISIL agent.”