Green activists who hijacked and occupied a power plant coal train have had their convictions quashed because the police withheld evidence from an undercover officer.

Prosecutors admitted that the trial prosecution was responsible for a "catastrophic failure of disclosure" to both judge and jury during the 2009 trial of the "Drax 29."

Heavily redacted files recording communications between undercover officer Mark Kennedy, known as UCO 133, and his police handler show that he was present for the protest briefing and that he drove members of the group to a level crossing near the power station.

The redacted documents also note that the group had conducted a "full thought-through risk assessment" before stopping the train, which was laden with 42,000 tonnes of coal.

"Those stopping the train were going to do so in strict accordance with rail track policy. All people involved are switched on and very safety conscious ... it was meticulous," the documents said.

Referring to the documents, the Lord Chief Justice Thomas asked repeatedly "on whose authority have these documents been redacted?"

He said: "It is not satisfactory that the police should decide which sections of documents to redact and which to make public. That should be a decision for the courts not the police."

The case is the third in which charges have been dropped or convictions overturned as a result of failures to disclose undercover police evidence, bringing the total number of campaigners denied a fair trial to over 50.

Appellant Robbie Gillett said, "In our trial in 2009, the police and the Crown Prosecution Service deliberately withheld evidence from the jury. They're not interested in providing a fair trial to the political activists which they spy upon.

"This is political policing. It is an invasion of people's lives, a waste of public money and from the police's perspective, a legal failure."

A London court will hear a case involving another undercover Metropolitan Police officer on January 27.

Jim Boyling is accused of misleading a court by appearing as a defendant under his assumed activist identity and using the same lawyers as the other defendants to gain access to confidential legal advice.