The undercover police officer whose seven-year infiltration of the green protest movement has sparked widespread controversy is said to have named another eco-activist as a fellow police spy, the Guardian can reveal.

PC Mark Kennedy is understood to have confirmed the woman was a fellow police officer two months ago, when being confronted by friends over his true identity.

Although Kennedy later quit the Met, expressing misgivings over an audacious operation that saw him betray close personal friendships, his identification of a fellow undercover operative amounts to a serious breach of protocol and is likely to add to the mounting concern over what appears to have been a coordinated operation to disrupt a peaceful activists' campaign against climate change.

Kennedy's double life under the fake identity "Mark Stone" was exposed two months ago by activists who had become suspicious of his willingness to help plan and pay for a planned invasion of a power station near Nottingham.

An investigation by the Guardian revealed he used a fake passport to travel to 22 different countries, gleaning information about left-wing activists and relaying sensitive details back to his police handlers since around 2003.

Today six activists walked free from Nottingham Crown Court after questions over the undercover officer apparently led to the collapse of their trial. A lawyer for the activists, who were among 114 arrested for conspiring to occupy the Ratcliffe-on-Soar power station in 2009, said that had Kennedy's role not been exposed, the operation could have led to a serious miscarriage of justice.

It emerged the undercover officer, who quit his job with the Met after expressing regret over his actions, offered to help with the defence's case. Mike Schwarz, a solicitor from Bindmans, said Kennedy had "gone native" after becoming convinced of the need to combat climate change.

In parliament, Keith Vaz, the chairman of the Commons home affairs select committee, said he would write to the Met commissioner, Sir Paul Stephenson, after questions emerged over the accuracy of parliamentary testimony about the use of plain clothes officers at protests. Vaz said it had appeared the commissioner and one of his commanders, Bob Broadhurst, had failed to disclose "the full facts" about the infiltration of protests when giving parliamentary evidence in 2009. "During our inquiry into the G20 protests, [MPs] explicitly asked Sir Paul Stephenson and commander Bob Broadhurst about the deployment of undercover officers," said Vaz. "I am disappointed they appear not to have given us the full facts."

Labour MP David Winnick said the home secretary, Theresa May, should address the allegation that the Met officer strayed from being a passive spy, and worked instead as an agent provocateur.

In Berlin, a German MP revealed he had tabled parliamentary questions after discovering the Met officer operated undercover "in German territories". The member of the German parliament asked how Kennedy was allowed into the country and alleged he had "sexual relations" with other activists while undercover.

Kennedy, 41, attended dozens of protests across the UK and Europe, gaining the trust of environmental activists while claiming to be a "freelance climber".

Working as a "driver" during direct action protests, the undercover officer obtained sensitive details about protests, and earned the nickname "Flash" because of his generosity with money, after helping fellow activists hire vans and pay court fines. However, Kennedy's friends became suspicious and discovered paperwork proving he was a police officer. Confronted in October, Kennedy confessed and expressed remorse over his actions.

Kennedy was then asked about other individuals in the protest movement about whom activists had suspicions. One was a campaigner who lived in Leeds and was closely involved in planning a major protest intended to close down the Drax coal-fired power station in North Yorkshire. There were already suspicions over the woman, who was in her 30s, after she disappeared suddenly around 2008 claiming to have fallen in love with a man in Coventry. The woman has not been seen since.

The six friends present when Kennedy broke down and admitted he was a spy then asked him directly if the woman was also a police officer. "He [Kennedy] nodded and said: 'Yeah, but you know about that already," said Craig Logan, 37, who was present. Kennedy is then said to have indicated that there were several other police officers living undercover in the protest movement.

Logan said that while there was circumstantial evidence suggesting the woman was operating undercover, Kennedy's former friends were highly suspicious of all the information he revealed that night. "This man was an extraordinary liar," he said. "We cannot take anything he told us at face value."

Tom Brake, the Liberal Democrat MP behind much of the questioning, has revealed doubts over the accuracy of evidence given by a third senior police chief.

Sue Sim, the deputy chief constable of Northumbria Police, but also in charge of public order policing for ACPO, told the committee she did not expect undercover officers were deployed.

Brake pressed her again in correspondence when she replied that her statement to the committee was "completely unequivocal".

"My response was that under no circumstances do I consider it appropriate to deploy plain clothes police officers into a protest of demonstration scenario."

In a tape recording during which Kennedy is questioned by another activist, broadcast last night on Newsnight, he is asked about the police use of infiltrators. "I'm not the only one by a long shot – it's like a hammer to crack a nut. It's spun in different ways but you know you start looking at the way the law is used and manipulated and well – fuck," he said.