Twenty-nine people convicted of offences after a climate change protest in North Yorkshire where a Metropolitan Police officer was working undercover are appealing.
The Court of Appeal hearing comes after an announcement in 2012 by the then Director of Public Prosecutions Keir Starmer that there were concerns about the safety of convictions which followed the Drax power station protest near Selby in 2008.
The actions of Mark Kennedy, who spent seven years posing as Mark "Flash" Stone, had led to the collapse in 2011 of the case against six protesters accused of planning to invade the coal-fired Ratcliffe-on-Soar power station in Nottinghamshire and led to a review of police undercover tactics.
A decision on whether or not the Drax convictions remain safe will be made during proceedings in London before the Lord Chief Justice Lord Thomas, Mr Justice Simon and Mr Justice Irwin.
Mr Starmer said he was inviting those convicted after the Drax protest to appeal after a review of the case by a senior CPS lawyer and after taking advice on the safety of the convictions from a senior QC.
He said at the time: "Having considered the conclusion of that review carefully, I have decided that the safety of the convictions should be considered by the Court of Appeal.
"That is because it appears to me that a senior CPS lawyer, who has since left the organisation, may not have complied fully with disclosure obligations in this case."
He said that as the prosecution cannot lodge an appeal in those circumstances, he had taken the unusual step of writing to the legal representatives of those convicted, "inviting them to appeal on the basis of non-disclosure of material relating to the activities of the former undercover officer, Mark Kennedy".
Environmental campaigners ambushed a freight train as it took fuel to Drax, the largest coal-fired power station in Europe.
Participants in the non-violent protest were sentenced at Leeds Crown Court in 2009 for obstructing the railway.
They were charged with obstructing engines or carriages on railways, which is an offence under the Malicious Damage Act 1861.
The Crown Court heard how the train was stopped by two men posing as Network Rail staff, wearing orange jackets and hard hats, who held up a red flag.
Moments later, the train and a nearby bridge were scaled by the protesters wearing white paper boiler suits and carrying banners.
The protest lasted 16 hours, causing delays to numerous freight and passenger services and the clean-up operation cost Network Rail nearly £37,000.
Defendants told jurors they did not believe they were doing anything criminal because they were trying to prevent climate change.
Some were ordered to do 60 hours of unpaid work and others were given conditional discharges for 12 months.