The man, named only as Saleh, said the masked militant who appears in several beheading videos was a senior figure in the extremist organisation responsible for murdering foreign captives.

Jihadi John, who was recently unmasked as 26-year-old Londoner Mohammed Emwazi, was last seen in a video posted online which appeared to show Japanese hostage Kenji Goto lying dead.

Saleh, who worked as a translator, also revealed that ISIS routinely subjected hostages to mock executions so that when their beheading did finally come they were not expecting to die.

Saleh said the prisoners would be given Arabic names to make it appear like they were with friends and continually reassured that they would not be killed.

He said: 'He would say to me "say to them, no problem, only video, we don't kill you, we want from your government [to] stop attacking ISIS.

"We don't have any problem with you; you are only our visitors".

Saleh said he explained to the hostages that they were safe, but always knew they would be killed.

His comments explain why so many of the prisoners appear so calm in the executions videos right up to the point where the knife is placed against their throat.