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.