The Crown Prosecution Service has launched an investigation after being accused of failing to disclose material in the case of undercover police officer Mark Kennedy.
A senior police officer from Nottinghamshire Police has written to the CPS with his concerns over the handling of the matter.
The allegation relates to surveillance tapes secretly recorded by Kennedy, according to The Guardian newspaper.
The CPS said there would be a "full and formal inquiry" conducted by Chris Enzor, Deputy Chief Crown Prosecutor for CPS North East.
Former Metropolitan Pc Kennedy spent a reported seven years posing as an environmental activist known as Mark Stone.
Six protesters accused of planning to invade Ratcliffe-on-Soar power station in Nottingham, the second largest in the UK, claimed prosecutors dropped charges against them after Kennedy offered to give evidence on their behalf, and their case was dropped.
The 20 previously convicted by a jury of conspiracy to commit aggravated trespass all escaped jail terms, and have since been invited to launch appeals against their convictions.
The Guardian claims that leaked documents show the CPS had been in possession of Kennedy's surveillance tapes for more than a year before the case was dropped in January.
The decision to discontinue the case had been made, the CPS said at the time, because "previously unavailable information that significantly undermined the prosecution's case" had come to light.
In an entry on its blog, the CPS said that the letter from Deputy Chief Constable Chris Eyre of Nottinghamshire Police contains "allegations about a failure to disclose material".