The following post has been taken from Indymedia. There is no reason to doubt it's authenticity. While it is sickening, it is no surprise to discover undercover police working in this way. There have been other accounts of such things happening.
Update, 25 October 2010: Fitwatch can now confirm the accuracy of following statement. Several other groups have also provided confirmation.
"Mark 'Stone' has been an undercover police officer from 2000 to at least the end of 2009. We are unsure whether he is still a serving police officer or not. His real name is Mark Kennedy. Investigations into this identity revealed evidence that he has been a police officer, and a face-to-face confession has confirmed this. Mark claims that he left the police force in late 2009, and that before becoming an undercover officer he was a Metropolitan police constable.
Please pass this information on to anyone who may have been in contact with Mark in the last decade, both in the UK and abroad."