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He was known to those within it as Mark "Flash" Stone, having earned the nickname because he always seemed to have more money than the other activists.
He lived a double life: as Mark Kennedy of the Metropolitan Police and as Mark Stone, green activist, based in Nottingham.
"We're not talking about someone sitting at the back of the meeting taking notes - he was in the thick of it."
Speaking about the Ratcliffe-on-Soar protest, Mr Chivers said: "Mark Stone was involved in organising this for months - they could have stopped it at the start."
Instead, Mr Chivers said the police officer helped recruit as many people as possible.
He also drove a reconnaissance party to the power station in his van and then hired a truck for the main protest, Mr Chivers added.
The activists' plan was to try to shut down the coal-fired power station for a few days as a protest against global warming.
Eighteen people were convicted over the Ratcliffe-on-Soar case last year
But in April 2009, when 114 people had gathered for a meeting at the Iona School in Nottingham, hundreds of police swooped on the building and arrested them all for "conspiracy to commit aggravated trespass".
The trial of six green campaigners has collapsed after an undercover policeman who had infiltrated their group offered to give evidence on their behalf.
The six were charged with conspiring to shut down the Ratcliffe-on-Soar power station in Nottingham in 2009.
The case was due to start on Monday, but was abandoned after Pc Mark Kennedy contacted the defence team to say he would be prepared to help them.
The prosecution subsequently dropped their case.