A British man identified in reports as the Islamic State (IS) executioner known as ‘Jihadi John’ feared he was a “dead man walking” after run-ins with security services before fleeing to Syria to begin his reign of terror, email exchanges with a journalist have claimed.

British prime minister David Cameron vowed to use all means at his disposal to hunt down militants such as ‘Jihadi John’ after the killer was unmasked as a Kuwaiti-born computer programming graduate from London.

Boris Johnson has accused an advocacy group in the UK of being apologists for terrorism after it said the Londoner believed to be Islamic State executioner Jihadi John had previously been harassed by the British security services.

Security services are facing mounting questions over claims that the British graduate believed to be murderous extremist Jihadi John was known to MI5.

Mohammed Emwazi, the Briton identified in reports as Islamic State executioner ‘Jihadi John’, has been described as “extremely kind” and “extremely gentle” by a man who has known him since 2009.