An MI5 agent was allegedly recruited by a terrorist mastermind in Syria and sent back to Britain in a plot to launch a devastating attack against his handlers . The agent was recruited by MI5 as an undercover informant within the radical Islamic community and he then travelled to Syria where he became part of a jihadist group . But he apparently double-crossed his British handlers and returned to London with plans to launch an 'insider attack ' in the capital . The Mail on Sunday has been aware of the alleged plot for more than a year but agreed to previous requests from the security services not to publish the story for 'national security ' reasons . Parts of the plot were reported in a story in The Mail on Sunday last year , in which we revealed that the jihadist , who has a British passport , had plans to attack Central London and that David Cameron had been briefed about the details . This newspaper is aware of the man 's identity and further details of the investigation but are not printing them for legal reasons . A long undercover operation was run by MI5 to try to identify linked individuals who might be travelling back from Syria to launch similar attacks but this investigation has now concluded , meaning we can now report the fact that he was a potential 'double agent ' . The revelations come as the terror threat in the UK continues to be 'severe ' but as MI5 faces difficult questions about its tactic of recruiting agents who are known Islamic extremists . Supporters of Mohammed Emwazi , the Islamic State executioner known as Jihadi John , have claimed that MI5 harassed him and similar allegations have been made by the family of Michael Adebolajo , one of the killers of Fusilier Lee Rigby . The security services refuse to answer such questions because they stick by a strict policy to 'neither confirm nor deny ' whether individuals have been approached or recruited – a policy they stick to even after suspects have been accused of serious crimes . However , The Mail on Sunday can disclose that the man worked for MI5 for a period before he decided to travel abroad in 2012 . MI5 faces difficult questions about its tactic of recruiting agents who are known terrorists , while supporters of Mohammed Emwazi ( pictured ) have claimed that MI5 harassed him . Investigators believe he met a terrorist mastermind with designs on attacking the West who encouraged him to plan an attack . Back in Britain , information suggested he began plotting to attack the MI5 handlers who had recruited him and set about trying to buy automatic weapons in this country or abroad . The alleged 'double agent ' was apparently planning to use the firearms to stage a murderous attack on the public in an echo of the Kenyan shopping mall massacre last year , sources said . He remained in contact with his jihadi controller , a terrorist mastermind who is thought to have established links to Britain . The agent also joined up with another extremist and they approached two other men for help in obtaining the equipment , but these other men did not agree to help them . MI5 were sufficiently concerned by an intercepted communication they received that they launched an emergency operation to apprehend the man . Scotland Yard 's firearms unit was sent to Central London when investigations suggested he was close to launching his attack . The man was detained for questioning and remains in custody . However , no firearms were discovered linking him to a plot and it appears the security services ' fears of an imminent attack were unfounded . There has been growing concern from within the security services that there could be 'blow-back ' from the conflict in Syria , where hundreds of young Britons have joined the fighting . Eleven months after the agent was allegedly planning his attack , police and MI5 rounded up another cell linked to IS also allegedly planning a gun attack in Central London . Five men are currently awaiting trial . Some details of how MI5 go about recruitment have been released in recent months as part of an investigation by the parliamentary Intelligence and Security Committee . They often approach individuals they are investigating in order to try to recruit them as agents , known as Covert Human Intelligence Sources . They then assess them on their suitability , their access to intelligence , personal qualities and their motivation for working with MI5 . Recruiting a target is seen as a 'positive outcome ' as it means they have reduced the risk , gained increased 'visibility ' of their activity and have a new source of intelligence .
An MI5 agent was allegedly recruited by a terrorist mastermind in Syria . He was then sent back to Britain in a plot to launch an attack against MI5 . An undercover operation was launched to try and track any similar plots . Covert Human Intelligence Sources has arrested the man and he remains in police custody .