Police have escaped misconduct proceedings for keeping an undercover officer secret during the collapsed prosecution of activists accused of plotting to shut a major power station.
"Collective failings" were to blame for Nottinghamshire Police not disclosing the role of undercover officer Pc Mark Kennedy, the Independent Police Complaints Commission has ruled.
The police watchdog found that "the actions of individual police officers and members of police staff did not amount to misconduct" during the Ratcliffe-on-Soar case.
Investigators found the sharing of sensitive information was badly handled by a number of parties. The case sparked widespread concern after it emerged former Scotland Yard Pc Kennedy spent years posing as an activist known as Mark Stone.
The IPCC report supported a Crown Prosecution Service inquiry which said both senior lawyers and officers were to blame for not declaring surveillance evidence during attempts to convict protesters in the case.
Len Jackson, a commissioner with the IPCC, said: "Our investigation has shown that the sharing and recording of sensitive information, initially between the various officers involved and then with the CPS, was not well handled.
"In particular, where the use of an undercover officer has been authorised, the police need to be meticulous in their handling and dissemination of any evidential material and in ensuring that liaison with the CPS is well documented. 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.
"We believe there is some learning from this case on all sides and we have discussed our report with Nottinghamshire Police and with the Association of Chief Police Officers.
"We note the recent findings of Sir Christopher Rose's report into the CPS handling of the case; that there were individual failings but no deliberate or dishonest withholding of information. I endorse the DPP's desire to see a memorandum of understanding between the police and CPS to guide their liaison in any such future cases."
Mr Kennedy went undercover to infiltrate left-wing protest groups, travelling to 11 different countries on 40 occasions. But the case was dropped against six protesters after Mr Kennedy offered to help the defence. A further 20 people had their convictions quashed after previously being convicted of conspiracy to commit aggravated trespass.