An undercover officer's secret role goading peaceful climate protesters to break the law was decried as a "step towards a police state" by Green MP Caroline Lucas yesterday.
Ms Lucas's comment followed the collapse of the trial of six people accused of trying to shut down Ratcliffe-on-Soar power station in 2009 - amid claims that a policeman who infiltrated their group had offered to give evidence on their behalf.
The case was due to be heard at Nottingham Crown Court yesterday, but was abandoned after the officer, named as PC Mark Kennedy, apparently contacted the defence team to say he would be prepared to help.
Kennedy, a former member of the Metropolitan Police, spent the past seven years undercover in a number of political and environmental groups.
He is understood to have subsequently resigned from the force and left the country.
Some activists have claimed that his role went beyond that of observer and that he helped fund the protest and planned to take on a main role in disrupting the power station.
Mike Schwarz, of Bindmans solicitors who represented the protesters, said: "I have no doubt that our attempts to get disclosure about Kennedy's role have led to the collapse of the trial.
"It is no coincidence that, just 48 hours after we told the CPS our clients could not receive a fair trial unless they disclosed material about Kennedy, they halted the prosecution."
The Crown Prosecution Service denied this was the case.
Earlier this month 20 protesters were sentenced to a mixture of community orders and conditional discharges after they were convicted of conspiracy to commit aggravated trespass at Ratcliffe.
They were among 114 pre-emptively arrested, including PC Kennedy, when police raided the Iona School in Sneinton, Nottingham, in April 2009.
Ms Lucas said: "The police infiltration of a non-violent protest group like this one is not only hugely expensive - diverting police funds away from tackling serious crime - but also takes us one dangerous step closer to a police state.
"That the undercover officer here was an agent provocateur rather than a mere observer raises alarming questions about the intentions of the police and ultimately the government. Such anti-democratic forces have no place in our society."
Police monitoring group Fitwatch said it feared other groups had already been or would be targeted in a similar manner.
Fitwatch activist Val Swain said: "We believe that undercover police officers are routinely used, not just to gather information on criminality, but to deliberately disrupt and undermine political dissent and protest.
"Undercover police have undoubtedly been used to deliberately weaken the organisational structures of protest groups, and to identify and target people who are key organisers.
"Of course there will be others like Mark, infiltrating protest organisations across the UK. After the student demonstrations, that could now include student groups and others who oppose government cuts. The impact that this could have on the effectiveness of these campaigns from now on is truly worrying."
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