Unmasked: The second member of ISIS's "Beatles" execution cell

It can be revealed that Kotey travelled to the Middle East alongside three other known extremists on a controversial aid convoy to Gaza organised by the London mayoral candidate George Galloway in 2009 – and friends in west London have not heard from him since.

He is the second member of the cell to be identified, after “Jihadi John” was exposed as west Londoner Mohammed Emwazi, who was killed by US a drone strike in November. The other members of the cell, nicknamed “Ringo”, “George”, and “Paul”, remain among the world’s most wanted men and are being hunted by intelligence and security services on both sides of the Atlantic.

A US intelligence official confirmed that Kotey had travelled to Syria and said his role in the taking of Western hostages was being investigated. A UK security official declined to comment.

It is not clear whether Kotey is the guard nicknamed “Ringo”, who has previously posted online about growing up in west London’s Shepherd’s Bush area, or “George”, identified by some hostages as a senior figure in the group. There are understood to be discrepancies in the accounts of freed hostages as to which guard had which nickname.