ALAN IS a car thief. He is a drug user, a football hooligan, a thieving barman and a chauffeur. A white male in his thirties, he looks like Lemmy out of Motorhead after an evening out with Johnnie Walker. He spent several months last year plotting and drinking with a group of known criminals in a Northern city; at one stage he was asked to help organise a serious assault.

Alan is also a police constable, and his bosses know everything he's been doing. He is an undercover officer with a regional police force. Alan - not his real name - has been in the force for more than a decade, and he's proud of what he does, blending in with criminals, sometimes for months on end, to help "catch the big boys". He says he wouldn't do anything else.

The mystique of the undercover cop, has recently been tarnished in a series of high-profile cases.

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Two black Scotland Yard undercover policemen are suing the West Midlands force, claiming their counterparts botched an operation in Birmingham in which they were gunned down and disabled for life.

`Lizzie James', the detective who pretended to woo Colin Stagg, once the prime suspect in the 1992 murder of young mother Rachel Nickell, by pretending to share his deranged sexual fantasies, is in the news again. Last week it was reported that she had quit the force due to stress and is preparing to sue the Metropolitan Police.

Since the case was thrown out of the High Court in 1994 (the judge labelled the police methods "deception of the grossest kind"), James is reported to have suffered psychological problems, gained weight, and lost interest in sex.

There has been no official comment on either of the cases. Some officers who have been involved in unrelated undercover operations are privately dismissive, saying the young officer has spotted a way of making money out of her former employers.

Undercover work spans a huge range of operations, from the risk-free to the potentially lethal. In an operation in Southampton earlier this year, a policeman had simply to walk into a travel agent and buy an air ticket to Majorca while mentioning he was an antiques dealer.

Right now, across the country, drug dealers, money counterfeiters, immigrant smugglers and hitmen are talking to people who they believe are fellow- criminals - policemen who risk death if one wrong move gives them away.

Officially there are no full-time undercover policemen; investigation units ask for volunteers or call in officers attached to other units whenever they need them. SO10, Scotland Yard's undercover unit, has only a few dozen officers working for it at any one time.

"The requirements vary as much as the type of work itself," says one source familiar with undercover work. "One force might need a native Kurdish speaker to blend in with some smugglers. Someone else might need someone who had trained as a BMW mechanic, someone else might need a jeweller or a jazz saxophonist or a chemist or a pizza chef. Or someone with a Geordie accent and an intimate knowledge of rave music who can drive a motorbike.

"There is no such thing as an `undercover policeman'; each job needs someone different." Sometimes those in charge have someone in mind, or on file; if not, the word gets sent out to different forces. "Some people do one undercover job that lasts two days in 20 years on the force; others are doing them more or less full-time," says the source.

It is those involved in the high-risk, long-term operations, that require the assumption of a false identity - so-called "level one" assignments, who receive the most intensive training.

Paul Britton, the controversial forensic psychologist who briefed Lizzie James, has also been involved in numerous other operations across the country. Britton, who described the Stagg case in his memoirs published last year, refuses to comment on James. But, he says, volunteers for undercover operations go through a rigorous selection mechanism.

"There are certain underlying requirements. You need someone intelligent and with social insight, able to react quickly in a given situation and adapt different qualities.

"In real-life, most of them are self-confident and gregarious, though the way they behave is inevitably affected by the nature of the operations they have done before. An ostensibly larger-than-life, outgoing man could be destroyed by the job, a small, meek woman could do very well."

Before going on dangerous jobs, the undercovers are subjected to real- life simulations to test how they would react in typical situations. The undercover source says inexperienced and macho policemen usually don't survive this stage. "It's very carefully run. You just don't take risks."

Cover-stories and alibis are painstakingly constructed. If an undercover officer has to tell his cronies that he was in the Royal Greenjackets, "either he actually will have been in the Greenjackets, or we'll make damn sure he'll be able to talk about units and events like he was there. He'll be able to describe the food and the colour of the carpets."

Normally, detectives won't risk using someone who has to spin such a risky yarn, but circumstances might dictate there's only one candidate suitable for the job.

The problem area is personal life. A typical undercover assignment might involve a policeman working as a driver and mechanic for a crime ring. Inevitably, he'll be asked about whether he has a wife and children. Informed sources told us about their mechanisms for dealing with these questions convincingly; we have chosen not to reveal them.

The stresses of the job can be chronic. Officers can live under assumed identities for months, socialising with people they are employed to arrest, aware that one false move can give them away. "We usually use our real Christian names," says Alan, "because of the possibility of recognition.

"If a friend recognises you in the street while you're with an undercover suspect, he'll come up to you and say, "Hi Alan." It would blow your cover if you used an assumed first name." Most officers who work undercover frequently tell their friends to walk past them whenever they see them in the street, even if they appear to be alone.

One policeman tells of the time a pounds 3 purchase gave him away: while pretending to be a wealthy arms-dealer, he bought a pair of rubber stick-on soles for the Gucci loafers he had been provided with. The next day, he met his suspect as planned in the Dorchester; after that meeting he never heard from him again.

Criminals are also increasingly aware of undercover penetration. Organised rings are known to watch police stations and keep a note of the registration numbers of unmarked cars, though most police operations are more sophisticated: one experienced undercover officer has been driving a pounds 50,000 convertible sports car for nine months as part of his identity.

Some undercover policemen, such as Lizzie James, are one-offs, the right people for the right job. But there isn't too much demand for beautiful young women who can pretend to have bizarre sex fantasies, and many of the men and women on repeat long-term operations have a maverick, almost bohemian side.

"It's not like being in the force," says Alan. "It gives you freedom and I love it." Many, though, end up being loners even if they didn't start out that way; pretending to be a crack dealer for nine months does little for a marriage.

It may sound glamorous, but it could also be lethal. Britton says supervising officers are - or should be - aware of a number of different types of stress: "The officer is all alone on the job. It's not too much consolation to know that if someone pulls a gun on him his back-up will come running over from three fields away.

"There is a constant fear, which officers tend not to recognise, as it manifests itself as arousal. It's the fear of discovery and also of letting the side down. Long-term investigations take a lot of money and manpower, all of that is riding on your shoulders and it tends to funnel down."

Often the undercover officer is on an adrenaline high: it's those in charge who order an investigation to be aborted because it's become too risky, above the objections of the officer involved.

Investigators say constant support is essential, to give the undercover officer a chance to release everything he knows, to be able to explain what is happening in a detailed and structured way. This is far more recognised now than it was even ten years ago.

Says Britton: "If you're under sustained pressure (without the chance of release), you may have a bit of a wobble at first, then it seems like everything is fine as a coping mechanism kicks in. But without the support of colleagues, your system can just crash, suddenly, with no warning."

Training officers and police psychologists stress the importance of constant support and debriefing, with some saying that a couple of weeks' rest between big jobs should be mandatory.

Jennifer Brown, a forensic psychologist at the University of Surrey and a specialist on stress in the police force, says that in some respects undercover officers lead less troubled lives than "ordinary" policemen. "The structures to support undercover work are in place and they are very thorough," she says.

"The most stressed people in the police force are traffic officers. They're confronted with constant low-level trauma (bodies in car accidents, fights) and they're expected just to cope with it." Policewomen, she adds, can have an even worse time, as the macho culture discourages their more natural tendency to cope with their emotions by articulating them.

Despite the controversial methods used at Paul Britton's behest - describing copycat fantasies involving group sex and simulated rape - Lizzie James had plenty of support while she was assuming her new identity as a perverted murderer - something she had volunteered to do.

But the case failed through no fault of her own, and the question that needs to be answered is whether she was adequately supported by her colleagues and superiors, or whether they could have saved her.