"The gangster pulled his car over on a dark country lane. He stuck the barrel of his gun into my mouth: 'You're a cozzer, aren't you?' Any hint of doubt in his mind and I would have been killed; my body dumped. Training had prepared me for this moment. I didn't flinch. He believed my denials and we went for a drink.
"In truth, I was a Met detective working undercover for Merseyside Police. They had a corruption problem involving officers working for Curtis 'Cocky' Warren's drug syndicate.
"A good UC [undercover cop] needs to develop several 'legends', which he can put on like a well-worn leather jacket and walk into the underworld with confidence. I was a fly businessman with horse racing interests. We entertained the targets in the Cocky Warren corruption operation in a private box at Haymarket. It was wired for sound and had other UCs posing as waiters.
"I was part of a small group of Level 1 undercover officers who the Met deployed across county and national borders in long-term infiltrations. I think I was the last UC trainee to be 'kidnapped' by Special Forces; put in a metal box and then taken out for interrogation. The Met stopped the practice for fear of causing psychological damage before recruits were ever sent into a criminal gang.
"There have been other harrowing assignments I can talk about now. I remember that to bug this paedophile's flat, I had to befriend him and pretend to get off watching kiddie porn. I also spent 18 months in a shop front sting. We'd set up a pawnshop in Bermondsey, south-east London to gather intelligence on a group of criminals. One of the people later convicted sent a message back to me later saying: 'No hard feelings.'
"Covert policing is the last bastion of pure policing: when it works, it should become, in court, an unassailable combination of eyewitness and covertly recorded evidence that greatly increases the chances of convictions. But as the gathering of intelligence becomes more political, UC work can also become corrupted and self-justifying. Behind the mystique and requirement for absolute secrecy, senior officers can lose focus and use the kudos of having someone on the inside to build their own careers or pursue a questionable agenda.
"There are many thorny issues about covert policing that a proper independent inquiry into the Mark Kennedy affair will have to grasp.
"I can understand how someone like Kennedy was deployed for so long. His deployment inside the protest movement is said to have started seven years ago. The timing is also important, because the Met had just been humiliated in court after another long-term infiltration was found to have spiralled out of control. In that case, Operation Cotton, the Met had allowed two UCs posing as money launderers on the Costa del Sol to operate for seven years. It cost tens of millions of pounds. I was one of the many UCs asked to go to Spain with cash to fund their high-rolling lifestyle.
"The targets were an allegedly corrupt network of drugs and tobacco smugglers working with bent Gilbraltarian politicians. But after a lengthy trial, the judge threw out the prosecution. He called it 'state created crime'.
"The judgment [in 2004] was supposed to have forced a major internal review of the oversight of long-term undercover operations. But the management lessons don't seem to have been learned in the deployment and handling of Kennedy. He now stands accused of having 'gone native' and, worse still, of compromising the identity of another UC. Was this a crisis of conscience? And should it have been spotted before Kennedy imploded?
"UCs are supposed to be regularly debriefed by their cover man: we called them 'uncles' in the Met. They get the intelligence, and because they are also UC officers they have the skills to act as an emotional support and spot any signs of breakdown or chinks in the 'legend'. We are also regularly sent to the department psychologist.
"There are things you can't tell your family about and then there are little things like late payments that – if they are not sorted out – make you angry and make you feel the organisation is undervaluing your sacrifice. Allowing any resentment to fester is a risk to the operation.
"One of the dangers is you become so enclosed in your undercover life. On one job, management was concerned for our well-being so they flew over Donnie Brasco to talk to us about how he felt after years undercover in the New York mob. He said: 'You won't be the same after this.' And I think that applies to Kennedy.
"Senior management don't always understand the immense burden of a double life. You coerce your family into the conspiracy. Of course, it's an adrenalin-fuelled journey. But undercover cops are the bravest people I've worked with and also the most unsuited to mainstream policing. It all comes at a price; drink and drug problems and failed relationships.
"There are different pressures on a UC infiltrating organised crime or terrorism, and someone like Kennedy, who was inside the protest movement.
"It's a lot easier to befriend and then betray a paedophile or cold-blooded killer than well-intentioned greenies whose worst crime, as far as you're concerned, is their dress sense, or music, and naivety. It also matters psychologically if you know that the risk of being found out simply amounts to having to finish your bowl of lentils.
"I understand how Kennedy may have become guilty about informing on innocent people. I struggled with what I called 'the peripherals', who hadn't done anything wrong but had come into our operation and would big themselves up about criminals they hardly knew, all on secret recordings. I remember thinking to myself when one of these 'peripherals' was talking: 'Shut up! Don't tell me this!' Because I knew they now could be pulled in and used as leverage. This was the stuff of nightmares and that's why you needed to be debriefed.
"The indications are that Kennedy was left in too long and this compromised a prosecution, but it also threatens the other cases where there have been convictions as a result of his work. Why didn't his management pull him out earlier, after he had introduced another UC? That is good practice, because it preserves the integrity of the intelligence already gathered; strengthens the case you are building; protects the UC's mental health and shows a duty of care to those being spied on.
"It's said that the police may now face prosecution from some of the women Kennedy is supposed to have slept with. At training school, it was drummed into your head that you are only limited by your imagination. The whole UC model in the police is taken from the spooks, where an agent sleeping with the enemy is condoned.
"The official Met line was 'don't do it', but unofficially it was condoned. I remember one senior detective saying to me, 'Have you embedded yourself in the community yet?' It was tongue in cheek, but I left with the impression that had I shagged around for intelligence, it would have been OK.
"In the end, I left the Met on medical grounds, in 2004: after more than a decade undercover, I was a broken biscuit. The vicious internal politics and pressures of my undercover life led to a suicide attempt and mental breakdown. I got better, changed my name and trained as an actor. Last year, I did Macbeth at the Globe and I'm co-writing a script about the dilemmas of UC work. My new life is not a million miles from the scriptless improvisation of my old, UC life."
Liam Thomas was speaking to Michael Gillard
Undercover hits and misses
Colin Stagg was wrongly accused of the horrific murder of Rachel Nickell on Wimbledon Common in 1992 following a discredited police 'honey trap' operation in which undercover woman officer got him to confess.
Erkin Guney was alleged to have offered an undercover policewoman £10,000 to have his late father's lover run over. However, the court heard evidence that Mr Guney was the target of an incompetent, police-inspired sting operation, in which a grave-digger, wired up to broadcast their conversation to police who recorded it, attempted to persuade him to commission the murder of his father's lover. Police even staged a pretend accident, to persuade him that the murder had been carried out.
Guney was cleared in 2009.
Keith Hall was acquitted of murdering his wife, at Leeds Crown Court in March 1994, after the judge dismissed a tape-recorded confession to an undercover police officer.
A paedophile gang was convicted last November, at Leicester Crown Court, following an undercover police operation that began at the end of 2008. Two married fathers, Abid Mohammed Saddique, 27, and Mohamed Romaan Liaqat, 28, were sentenced to 11 and eight years respectively, after being found guilty of a string of sex crimes against teenage girls.
Dustin Hunter, a corrupt officer with Surrey Police, was jailed for drug dealing and helping to plot a jewellery store robbery. He was snared after confessing to an undercover operative in meetings at a west London pub.
Colin Gunn, one of the country's most dangerous criminals, was jailed for life, in 2007, after police used undercover detectives to dismantle his Nottingham-based multimillion-pound empire of drugs, extortion and violent crime. Undercover officers were drafted in after corrupt police tipped off Gunn about raids being planned against him.