Background: How double life among environmental protesters unravelled

THE story of Mark Kennedy - or Mark Stone as he was known to environmental activists - became common knowledge following a collapsed court case.

Six people stood trial accused of trying to shut down Ratcliffe-on-Soar power station, in Nottinghamshire, earlier this month.

But the case was abandoned after Mr Kennedy, who it emerged had quit the Metropolitan Police, apparently contacted the defence team and offered his help.

The 40-year-old had infiltrated the environmental protest movement in 2000, and spent seven years at its heart.

He travelled to 22 different countries gleaning information and playing a frontline role in some of the most high-profile protests, including those against the G8 Summit in Gleneagles in 2005.

His double life began to unravel when he was suspected of tipping off the police about the plans to disrupt Ratcliffe-on-Soar power station.

The discovery of his real passport followed and he was confronted in a house, in Nottingham, by activists in October last year.

Undercover officers such as Mr Kennedy are believed to cost 250,000 a year to fund.