Updated

The tattooed environmentalist, known to fellow activists as "Flash" because of his supply of ready money, had a secret — he was an undercover policeman who had spent years infiltrating the movement.

At some point, Constable Mark Kennedy had second thoughts about his mission.

British prosecutors on Monday dropped charges against six environmental protesters after their lawyer said Kennedy had offered to help the accused.

Activists and politicians called for an investigation into the clandestine police operation, saying Kennedy had played a key role in organizing and encouraging the protest that led to the arrests.

The defendants' lawyer, Mike Schwarz, said the case raised "serious questions" about the role of the police.

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

The U.K. Taxpayers' Alliance said the case will prompt the public to question the police's use of funds for undercover operations, as Britain claws back from the recent recession.

"A staggeringly large sum of taxpayers' money has been spent keeping this officer undercover for a decade and members of the public will question whether this was worth it if this court case has now collapsed," warned Emma Boon of the Alliance.

Green Party politician Jenny Jones said she planned to ask the police commissioner how many officers are working undercover.

"With money so tight, the police have to pick their targets carefully, and picking on a bunch of non-violent environmental protesters seems like a clear waste of money," she said.

The defendants were picked up in a controversial sweep of 114 activists in 2009 and charged with plotting to shut down one of Britain's biggest power stations.

Their trial had been due to start Monday, but at the last minute, public prosecutors said new information had come to light that "significantly undermined the prosecution's case."

The Crown Prosecution Service said in a statement that there was "no longer sufficient evidence for a realistic prospect of conviction." The charges were formally dropped at a court hearing in Nottingham, central England.

Schwarz said the trial collapsed after attorneys pressed for information about the role of Kennedy, who spent several years inside the protest group. Schwarz said Kennedy had been "willing to speak to me with a view to assisting the defense."

"It is no coincidence that, just 48 hours after we told (prosecutors) our clients could not receive a fair trial unless they disclosed material about Kennedy, they halted the prosecution," he said.

The activists were arrested in a nighttime raid at a school near the Ratcliffe-on-Soar power plant, 120 miles (190 kilometers) northwest of London, in April 2009. They allegedly had planned to occupy the coal-fired power station in a protest against carbon dioxide emissions linked to climate change.

Campaigners condemned the pre-emptive arrest, saying the activists had planned a peaceful protest.

Twenty people have already been tried and given community-service sentences or conditional discharges. The rest were not charged.

"I represented 113 of those arrested," Schwarz said. "The 114th, we now know, was PC Kennedy, an undercover police officer."

The Metropolitan Police refused to comment on Kennedy's role. Activists told The Guardian newspaper that the long-haired, tattooed Kennedy — whom they knew as Mark Stone — had been involved in many demonstrations in Britain and across Europe.

They said he often drove activists around in his pickup truck, participated in reconnaissance of the Ratcliffe plant and advised on the best way to break in.

Bradley Day, 23, one of the activists convicted earlier, said Kennedy "was a very practical person and very trusted" within the movement.

He was exposed after fellow activists found a passport bearing his real name.

An opposition lawmaker said the government needed to answer allegations that Kennedy had "acted almost as an agent provocateur."

David Winnick said Home Secretary Theresa May should make a statement to the House of Commons.