The damning ruling comes as 20 activists were told that their convictions for conspiracy to break into coal-fired Ratcliffe-on-Soar power station in Nottingham had been quashed.
The three judges said there had been a miscarriage of justice as a result of prosecutors not disclosing to the defendants’ vital evidence about the undercover officer, who used the alias Mark Stone while he was with the group for seven years.
They said that Kennedy “was involved in activities which went much further than the authorisation he was given, and appeared to show him as an enthusiastic supporter of the proposed occupation of the power station and, arguably, an agent provocateur”.
The court said that there had been “significant” non-disclosure of material “which would have been supportive of the defence case” at trial. The material related to recordings and a statement made by Kennedy, who infiltrated the group.
Lord Judge said: “Something went seriously wrong with the trial. The prosecution’s duties in relation to disclosure were not fulfilled.
“The result was that the appellants were convicted following a trial in which elementary principles which underpin the fairness of our trial processes were ignored.
Kennedy appeared to be an enthusiastic supporter of the proposed occupation of the power station and, arguably, an agent provocateur.
“The jury were ignorant of evidence helpful to the defence which was in the possession of the prosecution but which was never revealed. As a result justice miscarried.”
As Stone, Kennedy was authorised to act undercover and to infiltrate extreme left-wing groups in the UK, making contemporaneous notes and recordings of meetings he attended.
The judge said: “By 2009 Kennedy, apparently convincingly, purported to be a supporter of the beliefs of those who later became involved in the plot with which we are now concerned.”
In January, Kennedy denied he had “gone rogue” and told The Mail on Sunday that there had been “a lot of mistakes” made in the case. He did admit to having relationships with two female activists and said it was wrong, but denied allegations that he used sex to gain trust and intelligence.