Undercover officer Mark Kennedy, who infiltrated an environmental group, brought about the collapse of a trial against campaigners who planned to shut down coal-fired Ratcliffe-on-Soar power station in 2009 when he agreed to give evidence on their behalf.

Mark Kennedy spent seven years living undercover within the hardcore of the environmental movement, using the fake identity Mark Stone.

However after listening to arguments about climate change, Kennedy came to believe that he was fighting for the wrong side and switched his allegiance from the police force to the green activists.

Before quitting the police force and moving abroad Kennedy told friends he felt the undercover operation amongst environmental activists was “wrong”.

The trial of six campaigners collapsed today, allegedly after the undercover cop contacted the defence team to say he was willing to give evidence for the activists. The judge agreed to enter not guilty verdicts against all six defendants who had been accused of conspiring to break into Ratcliffe-on-Soar power station.

Speaking after the hearing, their solicitor Mike Schwarz said that the police force had some serious answering to do, including its use of undercover officers.

“The police need to answer some serious questions about their conduct relating to protesters.”

“This is a serious attack on peaceful, accountable protest on issues of public and pressing importance like climate change.”

“One expects there to be undercover police on serious operations to investigate serious crime. This was quite the opposite,” Schwarz said.

David Winnick, a Labour MP and member of the home affairs select committee, has said he is concerned about Kennedy’s alleged active participation within the movement.

My concern is the manner in which it has been alleged that Kennedy acted almost as an agent provocateur. In these circumstances, I think Mrs May should come to the Commons and make a statement,” Winnick said.