Inside stories: The art of infiltration

The collapse of a major trial involving a police undercover agent sheds rare light on a dark world

Speaking from his secret sanctuary in America, PC Mark Kennedy begged a green activist for forgiveness after being exposed as an undercover police officer who had been busily reporting the plans of his environmental campaigner colleagues to Scotland Yard for seven years.

In his broken state, the man who had led an elaborate double life as the impassioned activist Mark "Flash" Stone, left his former comrade with a grave warning: he was far from the only state agent currently spying on Britain's protest movement.

While PC Kennedy was a rare example of a covert officer "going native" – telling those he had betrayed that he wanted to make amends – police forces admit that long-running covert stings have always been an important part of their armoury.

From Edward Chapman, the wartime double agent codenamed "Zigzag" – so trusted by the Germans that he became the only British citizen to be handed the Iron Cross – to the exploits of intelligence officers Kevin Fulton and Freddie "Stakeknife" Scappaticci, who filtrated the IRA, it has been an experience that tests the skills and sanity of the officer.

PC Kennedy is full of regret. Others have suffered worse psychological torment. David Corbett's work as an undercover policeman in gangs across Britain led to dozens of arrests and convictions. But the pressures drew him to the brink of alcoholism and violence – he smashed up his own home. "People talk about living a double life. I was actually living three lives," he said after finally revealing his covert work. "I lost all sense of who the real David Corbett was." Eventually, he retired with post-traumatic stress disorder and weaned himself off alcohol. He had undergone extensive psychological testing before being allowed to take on the assignment.

In the case of PC Mark Kennedy, it is not hard to see why he was chosen: with his long hair, goatee and tattoos, he already looked like the experienced activist he would profess to be. But experts say that it is not always the obvious candidates chosen to become undercover agents. "Actually, they tend to prefer people who are blank slates and then mould them into a particular position," said Tony Thompson, an investigative journalist who specialises in telling the stories of former undercover police officers. "So they use less of the person's original personality than you might think."

The decision to use malleable officers was shaped by bitter experience, not least the security services' dealings with Edward Chapman, a noted safebreaker and crook before being hired by MI5. Minders soon realised they could not control their volatile charge: he spent wildly at Smokey Joe's and the Shim Sham Club in London's west end, noisily declaring exactly where his money came from. He was never used by the secret service again and eventually charged under the Official Secrets Act.

PC Kennedy is not alone in being rumbled. A misplaced passport led to his downfall. Though he faced the acrimony of being confronted by former friends, he was spared any threat to his life. Others have not been so lucky. Two undercover officers posing as crack cocaine dealers were shot by Birmingham Yardie gang members during a major drugs deal in 1994. They had been drafted in from the Met's SO10 undercover squad, now called the Covert Operations Group.

The two officers were later compensated after arguing that huge blunders had placed them in danger. The drugs money had been stamped with the words "West Midlands Police", they said, and their arrival in a Ford Mondeo, rather than a luxury car, aroused suspicions immediately. One officer "still had nightmares" seven years later.

Security sources say that psychological support for officers going undercover has improved in recent times after some high-profile disasters.

In one recent case, the would-be agent had no one to blame but himself for being quickly unmasked by the green group he was trying to infiltrate, Plane Stupid. Those who met "Ken Tobias", whose real name is Toby Kendall, saw him as "more Johnny English than James Bond". His clichéd Palestinian scarf and baseball cap sat oddly with his Armani jeans. His back story did not check out. Worse, he was always early to meetings. They soon unearthed his true identity and that he was hired by a private security firm. They fed him false information for a while, before finally being confronted.

The problem for the new wave of protest groups is that members use Twitter and Facebook to communicate and organise, and there is little central control.

Undercover agents

Name Eddie Chapman

Codename German: "Fritz". English: "Zigzag"

Undercover Two years

What happened? Offered his services to the Nazis while in prison in Jersey in 1942. He had fled to the island to avoid a safebreaking conviction in Glasgow. Dropped into Cambridgeshire in 1942, given task of blowing up a factory. Began to work as a double agent for MI5 but left the pay of the British secret service after the war and lived the high life in for a while in London. Ended his days running a health farm in Hertfordshire.

Name Toby Kendall

Codename Ken Tobias

Undercover 10 months

What happened? Went to meetings of the Plane Stupid group in July 2007, but aroused suspicion for always being early. True identity discovered online.

Name Freddie Scappaticci

Codename "Stakeknife"

Undercover 1978-?

What happened? Built a fearsome reputation as a member of the IRA's internal security unit, known as the "Nutting Squad". Implicated in murders of informers while employed by the state.

Name Mark Kennedy

Codename Mark Stone

Undercover Seven years

What happened? Began to work undercover in the green movement in 2003, becoming a senior figure. Went to dozens of other countries as an activist. True identity discovered by a girlfriend.