If someone tells you they were an undercover policeman, you might be sceptical. If they also claim to have been a property developer, actor, director of a commercial airline and a stand-up comedian, it becomes hard to hide your disbelief.

Yet this is the true story of James Bannon’s career.

We meet at Gardner Street venue Komedia to discuss that first chapter of his life when, as a newly qualified, 21-yearold policeman, he infiltrated The Bushwackers, a gang of Millwall FC-supporting football hooligans.

His story inspired a film – 1995’s ultra-violent ID – but it’s only now that he’s identified himself as the officer at the centre of it in his new book Running With The Firm.

For two years, Bannon lived a double life as Jim, a painter and decorator from Wandsworth who drank and fought alongside real-life Millwall fans on the infamous terraces of the 1980s. Tasked with gathering intelligence on those involved in the horrific violence of the era, he fabricated an identity that allowed him to become one of the gang and, in the process, almost started to believe his own lies.

He’s picked a pertinent time to tell his story; after the scandal of the Metropolitan Police stealing the identities of dead children and the publication of explosive revelations about undercover officers who married and had children with “targets”, public interest in this secretive world is at a high.

Bannon is critical but warns that it’s not easy for those on the outside to understand the life of an undercover officer.

He likens his experience to a two-year improvisation.

“You can run through any number of scenarios in the training room but on the ground you don’t always react the same way. You’re making split-second decisions and every decision you make has a lasting impact. Of course officers have to be held accountable for wrongdoing. You’re put into an environment and trusted implicitly to do the right thing and I don’t think some of these people did the right thing. But it’s a very fine line between what’s acceptable and what isn’t.”

In the book, Bannon describes a match against Middlesborough when Millwall fans had been pelted with rocks and pieces of metal and the police failed to intervene. When he got involved in the brawling it wasn’t to maintain his cover, it was to get revenge.

Does he feel he crossed the line that day? “I certainly wasn’t perfect and I’m not proud of some of what I did.

But at the same time there was a reason things happened and I feel I could justify them.”

He couldn’t, however, justify a relationship with an attractive barmaid whose father-in-law Bannon was surveying and he had to put a stop to their flirtation.

“That would have been a last resort. But I’ll be honest and say that if I absolutely had to, I would have. You do whatever you have to.”

In among the violence and deceit, there are some blackly comic episodes. Bannon and his partner were woefully underprepared for the roles they took on. At the beginning of the operation, his partner thought Milwall player John Fashnu was white – and nearly blew their cover saying so – while Bannon had to pretend to be illiterate to protect himself from accusations that he was “Old Bill”.

“When the operation was only going to last six months that made sense; over the course of two years it becomes pretty hard to keep up.”

Towards the end of the operation he got involved in a dare that involved him pretending to reveal himself as an undercover copper to the man he’d just bought a bag of cocaine from. His partner failed to appreciate the irony.

“The stakes were high but I can’t deny it was an exciting time. When you have a big ego like I do and are as arrogant as I was, it’s a job that comes quite naturally.”

But the operation ended on a whimper when Bannon and his team were told over the phone that the operation was being terminated and their services were no longer required. No arrests were ever made as a result of the intelligence they gathered.

At the time, Bannon admits it was a galling outcome. On reflection he says he is quietly relieved. “Arrests would have meant the animosity towards me was worse than it is.”

As things stand, he has only bumped into one of the people he surveyed since the release of the film. “He wasn’t overly happy about it but he took it well. He accepted I was just doing a job and 25 years on, no one’s in prison; no one died. But I hope the book demonstrates that it wasn’t work I went into lightly. My attachments to some of the people I met were genuine and we were as damning about bad behaviour on the part of the police as we were about hooligan violence.”

Bannon could have pursued his career with the police but on advice from a colleague chose to get out. “He told me that if I could convince all those people I was someone I wasn’t, it wouldn’t be hard to convince a bank manager to lend me money to start a business, or employees to come and work for me. In two years I’d worked in an environment that 99% of police officers never have the chance to do and it was time to move on to the next challenge.”

It wasn’t easy. “I could have wallowed in self-pity for ever but the thing I think I’m most pleased about is that I went through this extraordinary experience but it didn’t come to define me.”

In the years since, he moved to Brighton, trained as an actor, bought and sold a lucrative property company and even set up a commercial airline – not, he admits now, his greatest move.

“Richard Branson I am not.

But it taught me two things: always stick to what you know and make sure you use someone else’s money.”

In recent years he has become a stand-up comedian – a no-brainer really.

When he tells audiences he used to be an undercover policeman and once managed his own airline they do what any reasonable person would do and laugh.

* Running With The Firm by James Bannon is out on August 15 (Ebury Press, £14.99)