Drax campaigners have convictions quashed as authorities failed to reveal an undercover policeman had driven them to environmental protest

Lawyers could have argued that the demonstration might never have happened if it had not been for officer Mark Kennedy, who infiltrated environmental groups for seven years

Twenty-nine campaigners have had their convictions for ambushing a train in 2008 quashed because authorities kept it secret that an undercover policeman had driven them to the environmental protest.

The train was taking fuel to Drax near Selby in North Yorkshire, the largest coal-fired power station in Europe. Judges ruled there had been a failure to pass over information that would have allowed the activists to have a fair trial.

The undercover officer Mark Kennedy was the van driver taking the campaigners to the demonstration. He kept notes that were passed to senior officers via his handler – but police or prosecutors kept them secret, Court of Appeal judges heard.

Lawyers for the defendants could have argued that the demonstration might never have happened if it had not been for Mr Kennedy, who infiltrated environmental groups for seven years.

During the protest coal was shovelled on to the track to stop the train. It lasted 16 hours, and the clean-up cost Network Rail £37,000. Participants were sentenced at Leeds Crown Court in 2009 and 2010 for obstructing the railway. Some were ordered to do unpaid work and others given conditional discharges.

Lord Chief Justice Lord Thomas said: “There was a complete failure to make a disclosure fundamental to the defence … this court has no alternative but to quash the convictions.”