Police were facing serious questions today after using an undercover officer to infiltrate climate campaign groups for up to 10 years at a cost of hundreds of thousands of pounds.

The trial of six protesters accused of conspiring to shut down the coal-fired Ratcliffe-on-Soar power station in Nottinghamshire in 2009 collapsed after Pc Mark Kennedy offered to assist their case, although the Crown Prosecution Service (CPS) denied the link.

Defence lawyer Mike Schwarz said the officer's identity was only discovered after he was confronted by campaigners and added that if his evidence had been kept secret there could have been a serious miscarriage of justice.

He said the case raised concerns over the role of an officer as "agent provocateur", called for an inquiry into "murky" police tactics, and questioned the necessity of spending vast amounts of money and resources infiltrating a "peaceful, accountable and democratic" group.

Mr Schwarz said: "My clients were not guilty. They did not agree to join in any plan to occupy the power station. The evidence of Pc Kennedy presumably confirmed this. Yet that evidence, had it been kept secret, could have led to a miscarriage of justice.

"The police need to answer some serious questions about their conduct relating to peaceful protesters."

He questioned whether it was reasonable to incur costs of hundreds of thousands of pounds to infiltrate "peaceful, accountable, open, democratic" protesters, adding: "These are key questions about the budgets and accountability of the police, particularly when the police seem to be unhappy about answering any questions about their policy on undercover police officers and peaceful protesters.

"One expects there to be undercover police on serious operations to investigate serious crime. This was quite the opposite. This was civil disobedience which has a long history in this country and should be protected."

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 initially arrested, including Pc Kennedy, when police raided the Iona School in Sneinton, Nottingham, on the morning of Easter Monday April 13, 2009.

The protesters planned to trespass at the coal-fired Ratcliffe-on-Soar power station and shut it down for a week, a trial at Nottingham Crown Court heard.

The CPS said the new information which led to the collapse of the trial was "not the existence of an undercover officer".

In a statement, the CPS said: "Previously unavailable information that significantly undermined the prosecution's case came to light on Wednesday 5 January 2011.

"In light of this information, the Crown Prosecution Service reviewed the case and decided there was no longer sufficient evidence for a realistic prospect of conviction."

Mr Schwarz said Pc Kennedy and the new material were "obviously connected", adding: "These events beg wider serious questions. Would this evidence have been uncovered had the defence not become aware of it through other avenues?

"Is it appropriate that access to and decisions about disclosure of key evidence should exclusively be in the hands of a prosecution whose primary function is to secure convictions?"

He said Pc Kennedy had been "willing to speak to me with a view to assisting the defence" and appeared to have been persuaded by the arguments on climate change and the "appropriateness" of civil disobedience to tackle it.

Experts said the operation would have cost hundreds of thousands of pounds.

The Met Police said it was "not prepared to discuss" an undercover officer.

Danny Chivers, a defendant in the collapsed second trial, said the Met officer was confronted last year by environmental campaigners who had become suspicious.

He said: "We immediately realised that there was someone who could support our defence, who could say actually yes, most people who came to that school did not know the plan in advance. And yet the prosecution have never mentioned the fact they had this crucial evidence that backed up our case."

Senior backbencher David Winnick, a Labour member of the House of Commons Home Affairs Select Committee, said: "The concern is not the fact that the Metropolitan Police, and possibly other police forces, use undercover agents. No-one is so naive as to believe that that hasn't been the case since time began.