Police have been criticised for their role in withholding crucial surveillance tape recordings made by undercover officer Mark Kennedy.
The tapes were kept from activists who were being prosecuted for planning to occupy one of Britain's largest power stations. The contents of the tapes would have cleared the activists.
In a report published on Wednesday, the Independent Police Complaints Commission said "there was a failing by the police officers and police staff members involved to disclose" the tapes appropriately.
The IPCC began investigating last year after the prosecution of six activists collapsed.
The IPCC said there were collective failings by relevant parties to ensure the tapes were properly disclosed to the activists' lawyers but "the actions of individual police officers and members of police staff did not amount to misconduct".
The IPCC commissioner Len Jackson said: "Our investigation has shown that the sharing and recording of sensitive information, initially between the various officers involved and then with the Crown Prosecution Service, was not well handled … Whilst there were some weaknesses in the manner in which Nottinghamshire police officers and staff carried out their disclosure duties in this case it is our view that none of their actions amount to misconduct."
Kennedy, who infiltrated the environmental movement for seven years using the alias Mark Stone, covertly recorded a private meeting of activists on a £7,000, specially adapted Casio watch.
Nottinghamshire police used the intelligence to arrest more than 100 activists, hours before some of them planned to invade Ratcliffe-on-Soar power station, Nottinghamshire, in April 2009.
Twenty other activists were convicted in December 2010 but their convictions were overturned last summer when appeal court judges ruled that the Kennedy tapes had been withheld from them.
The IPCC report follows a similar inquiry by Sir Christopher Rose, a retired high court judge, who ruled in December that both prosecutors and police had failed to ensure the surveillance recordings made by the undercover police officer were handed over to lawyers representing the activists.
