Three senior judges have ruled that the undercover police officer Mark Kennedy unlawfully spied on environmentalists and arguably acted as an "agent provocateur".
In a damning ruling explaining why they quashed the convictions of 20 climate change activists, the appeal court judges said they shared the "great deal of justifiable public disquiet" about the case.
The judges, who included the lord chief justice, said "elementary principles" of the fair trial process were ignored when prosecutors did not disclose evidence about Kennedy's work to activists' lawyers.
There remain questions over his legal authority to spy on activists. Kennedy was not authorised to take part in the occupation of the power station until 9 April, just three days before the planned protest.His surveillance of the activists also exceeded his powers, according to the judgment.
"When the protesters started to congregate together just before the proposed occupation it appears that Kennedy went much further than his authorisation.
"That included undertaking reconnaissance, participating in briefings, checking the surrounding area for police activity and agreeing to take part in a team of climbers."
The judges agreed that Kennedy was arguably seen by younger activists as an "eminence grise" – influential decision-maker – in the group.
The judgment added that Kennedy played a "significant role in assisting, advising and supporting ... the very activity for which the appellants were prosecuted".
That finding could prove damaging for senior police responsible for the surveillance operation.Vera Baird, the former solicitor-general, said: "It was an ill-thought-out campaign to undermine people who turned out to be honest campaigners, not criminals, during which they wasted an enormous amount of money on this man who inevitably went native living with decent people for all those years.
"They were then left with him having let them down and with evidence showing that there was no crime in the first place."
Undercurrents is an award winning alternative news service producing videos of people taking inspiring actions. http://www.undercurrents.org