Media playback is unsupported on your device Media caption Former MI5 officer "Tom Marcus" tells Victoria Derbyshire how he disguised himself as a homeless person to foil an imminent terror plot

As an MI5 agent "Tom Marcus" risked his life for more than eight years, secretly tracking Islamic extremists and IRA dissidents on the streets of Britain. But when he left MI5 - with a CV full of inexplicable holes - he found himself taking jobs in a call centre and burger bar.

Tom Marcus is still jittery three years after retiring as a spy, he tells the BBC's Victoria Derbyshire programme. Walking down the street, he sometimes sees suspicious behaviour where there is none - shady people lurking in doorways; unusual bulges in coats; nervous, furtive glances.

Tom - not his real name, it can't be used for security reasons - saw plenty during his eight years as an MI5 surveillance officer to make him suspicious.

On one occasion he helped thwart a plan to blow up two coaches full of schoolchildren returning from a trip to France.

Undercover as a homeless man, and positioned near a mosque, he had noticed that a young Muslim man he had been tracking - who had fought with militant Islamist group Boko Haram in Africa before returning to the UK - had entered the place of worship but not left it. He had also counted more women leaving the building than going in.

He decided that one of the "women" leaving must have been the male extremist disguised in a burka. He informed his superiors, and the man - found to have six home-made bombs in his car - was arrested. The bombs were all set to go off at the same time via a mobile phone found on the suspect, meaning Tom almost certainly saved lives that day.

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The Victoria Derbyshire programme is broadcast on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.

Watch the full interview with Tom here.

The biggest operation in which Tom was involved stopped a planned bombing of a shopping centre in Manchester over an Easter weekend, potentially saving hundreds of lives. This was one part of a highly co-ordinated attack on people in the UK and US, directed from Pakistan, which also aimed to explode car bombs at the site of the Twin Towers on the anniversary of the 9/11 terrorist attacks.

Tom himself became a target for an attempted beheading when extremists developed a plan to kidnap MI5 operatives in the UK. The extremists had assigned people to watch the spies while they were following a suspect, and had covered the floor of a house in Kentish Town, north London, with plastic sheeting in anticipation of taking one captive. A black flag, three butchers' knives and a video camera, suggesting it would be filmed, were also found at the house.

Tom - who has written his memoirs, Soldier Spy - says that were it not for the extremists' "targets team" backing out at the last moment, he would not be here to tell the tale.

CV gap

In the end, the job caught up with Tom. He suffered nightmares about being attacked and was diagnosed with post-traumatic stress disorder (PTSD), returning to civilian life in 2013. He feels he's recovered now.

He also struggled to find another job. Because of the anonymity required by MI5, Tom, who joined the Army at the age of 16 before moving on to the security services, has a large gap in his CV.

"It's been hugely difficult to get a job," he says. "Working as an MI5 surveillance officer is seen as a job for life - so when you have to come out it's very difficult to figure out what job you can do.

"You can't answer the question about what you've been doing for the last 10 to 15 years in a job interview properly because you'd be breaking the Official Secrets Act."

Image caption MI5 headquarters in London

"Some people come up with a Ministry of Defence cover story," he says, "but trying to explain the skills you say you have for a made-up story just unravels - and I wouldn't feel comfortable doing that.

"So, work-wise, I just started at the bottom of the pile again. I got an MI5 pension but I had to make money by working in call centres and flipping burgers to build up work experience."

He is now a full-time writer.

"The only really well-paid options for someone like me were in places like Iraq and Afghanistan, but I just couldn't risk my life doing that.

"It was never an option. I didn't want my PTSD to come back. And no amount of money is worth risking being caught and beheaded."

'Mr and Mrs Smith'

Part of the reason for Tom's concern is that, while working as a spy, he married a fellow special operations agent and they had a son together.

"People used to joke about it, calling us Mr and Mrs Smith," he says, referring to the married spies in a film of the same name, starring Brad Pitt and Angelina Jolie. But, unlike the Hollywood actors, Tom's marriage has endured.

Tom, who grew up in poverty in northern England - his father an alcoholic ex-soldier who killed himself - takes extra care to make sure his son is safe at all times. He had the boy's clothes tagged so that he could be traced via GPS if he was captured on the way to or from his nursery. His sleeping bag at home is tagged for the same reason.

But, despite these precautions, Tom thinks the UK, which has not seen a multiple-fatality terrorist attack since the London bombings of 7 July 2005, is "the safest place in the world".

It is, he says, served by the "best group of intelligence agencies in the world" in MI5, MI6 and GCHQ. But they face growing and varied challenges.

Image copyright Gareth Cattermole Image caption 52 people lost their lives in the 7 July London bombings

"The tactics change completely," says Tom - who monitored suspected spies from Russia and China, and spied on the Continuity IRA, during his career.

"When you are watching an IRA terrorist you have to be careful about having no kit that could identify them. They're very good at counter-surveillance so could spot an MI5 officer unless they were very careful.

"Islamic extremists are normally quite settled in their local communities - and so the MI5 officers need to be very aware and sensitive about how they move in and out of these local areas so as not to draw attention to themselves.

"The Russians are so aware and in tune to surveillance operations from other secret services. If they suspect they're being watched they just won't do anything to do with their intelligence activities. You have have to watch Russian spies from a distance and never have the same people watching them.

"There is no one target who's easier or more difficult to watch - it's just different depending on who you are watching."

Of course, Tom says, it only takes one successful terrorist to create havoc on a grand scale.

"But if you do get frightened watching the news," he adds, "just be confident that everything is being done to keep you safe."

Tom Marcus's account of events has been vetted and cleared for publication by MI5 as part of his memoirs. Names and some specific details in the book have been changed in order to protect colleagues and ensure current and future investigations are not jeopardised.

The Victoria Derbyshire programme is broadcast on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.