A former MI5 spy has claimed the security services have so few ethnic minority staff, team leaders told agents to ‘black up’ while on a training exercise.

The former intelligence officer, codenamed Robert Ascott, said that he had worked for the agency for 18 years but broke his silence with an interview on Newsnight after being fired from the organisation.

The 46-year-old said he was aware of only one Muslim surveillance officer in the unit. "(MI5) was totally understaffed. We weren't used to dealing with the way they acted,” he claimed.

"The Islamists would meet round each other's houses or in mosques which you can't get away with going into. Also, they were living in mostly ethnic areas and often you would find the only white people on the street were surveillance officers."

Mr Ascott also revealed that MI5 staff were so stretched that British Transport Police had to be drafted in to follow a suspect during a major terror plot, adding that had the plot succeeded it “would have eclipsed 9/11.”

Although there have been a number of high-profile former MI5 members who have spoken publicly, Mr Ascott is the first ‘foot soldier’ who has chosen to reveal the daily grind of working for the British security services.

Mr Ascott, who is reportedly suffering from Post Traumatic Stress Disorder and panic attacks as a result of his work, was dismissed five years ago after he accidentally left an instruction manual in his garden shed. It was found and handed into police by a member of the public.