His extraordinary eight years as an undercover officer infiltrating the secret world of Britain's anarchist and eco-activist movements have generated headlines around the world and caused a furious debate over the policing of protest groups.
This weekend Mark Kennedy finally came out of the shadows to tell his own side of a story. In an interview with a Sunday newspaper, Mr Kennedy spoke of his time as the undercover police infiltrator Mark Stone and revealed that he now fears for his life.
"I can't sleep, I have lost weight and am constantly on edge," he said, speaking from the United States. "I barricade the door at night with chairs at night. I am in genuine fear for my life. I have been told that my former bosses from the force are out here in America looking for me. I have been told by activists to watch my back as people are out to get me."
But some of those fooled by Mr Kennedy denied yesterday that his life was in danger. While there was considerable anger at both his deception and the police's use of undercover operatives, they argue that overall the eco-activist community had acted responsibly, deliberately keeping certain information off popular web forums that could have been used to track the former police officer down.
"Any suggestion that there has been a witch hunt is ludicrous," said one activist. "I think generally the response has been very mature. A mainstream newspaper yesterday published a photograph of Mark's former wife. That's not something that we put up online."
On eco-forums used by activists to reveal the identities of two further infiltrators from the police's secretive National Public Intelligence Unit, debate has raged between those who believe Mr Kennedy and his family's whereabouts should be published, and those who have tried to keep such details secret.
"The fact that he's a scummy bastard who has no compunction about exposing his kids and ex-partner in that way, doesn't mean we should have done the same, does it?" argued one commentator yesterday. "Are we supposed to behave like police low-life and trample over people we don't know just to serve our ends?"
But there are elements within European anarchist and eco-activist movements who wish harm towards any police infiltrators. Yesterday the Anarchist Black Cross movement in Berlin, a city in which Mr Kennedy is known to have spent time undercover, issued a statement calling on English activists to make the former officer's life "unbearable".
Last month German anarchists uncovered a police informant who had been active in the town of Heidelberg and published a detailed online report about him. The group said yesterday: "This made us very happy and we think activists in England should learn from this example. To finish we hope that Mark, as an English policeman once said, spends the rest of his life looking over his shoulder. That is the minimum price he should have to pay."
Mr Kennedy's interview with the Mail on Sunday was the climax of a bidding war between news organisations to get exclusive access to a man who has shone an uncomfortable spotlight on the police's use of undercover operatives to access protest movements that use direct action. It is believed a fee was paid for his interview.
The police have defended their use of undercover officers as a vital method to gather intelligence. But many in activist communities have accused them of merely being "agent provocateurs".
The Independent Police Complaints Commission is now investigating whether senior officers tried to cover up Mr Kennedy's role after a trial of six demonstrators for conspiracy to commit aggravated trespass collapsed when prosecutors dropped the case.
The demonstrators' legal team claimed the decision was made after Mr Kennedy had a crisis of conscience and offered to give evidence on their behalf.
But there have been suggestions that the trial really collapsed because the Crown Prosecution Service discovered Nottinghamshire Police had withheld secret recordings of the demonstrators. Mr Kennedy's interview now lends extra weight to those accusations.
"The truth of the matter is that the tapes clearly show that the six defendants who were due to go on trial had not joined any conspiracy," said Mr Kennedy, claiming that he was the officer who made the secret recordings. "The tapes I made meant that the police couldn't prove their case. I have no idea why the police withheld these tapes."Reuse content