Six environmental protesters were convicted after they were denied the ability to put a “reasonable excuse” defence or climate facts before the jury, despite these being afforded to other activists acquitted for taking part in the same demonstration.

After an eight-day trial at Southwark crown court in London, the six Just Stop Oil (JSO) activists were found guilty of public nuisance, which carries a maximum 10-year sentence, for climbing gantries on the M25 in 2022 to demand an end to new fossil fuel projects. They will be sentenced next month.

The way their case was handled contrasts starkly with that of three other JSO activists who took part in the same demonstration on London’s orbital motorway.

They were found not guilty of public nuisance after the judge at Guildford crown court allowed them to argue a defence of reasonable excuse and prosecutors permitted them to include 12 climate facts in the agreed facts – undisputed by both prosecutors and defence lawyers – presented to the jury. The verdicts in the two cases were less than three weeks apart.

Adelheid Russenberger, a history PhD student from London, who is one of those being sentenced at Southwark crown court next month, said: “It was just a complete disparity in how the judges treated the case and, to an extent, how the prosecutors dealt with them.

“One previous prosecutor was happy to accept some agreed facts regarding climate and the other wasn’t.”

The introduction of the statutory offence of public nuisance under the controversial Police, Crime, Sentencing and Courts Act (PCSCA) was widely criticised as an attack on the right to protest.

Section 78 of the act includes a defence of reasonable excuse, but Judge Perrins, sitting at Southwark, refused to allow Russenberger, who represented herself, and her fellow accused to advance the defence to the jury.

He ruled: “The subjective belief of each defendant that their conduct was justified as an act of protest cannot afford them a reasonable excuse under the terms of the act for their subsequent actions.” He said that would prevent any protesters being convicted of public nuisance.

Raj Chada, head of criminal defence at Hodge, Jones and Allen, who represented Andrew Dames at Southwark and Isabel Rock at the Guildford trial, said: “There is an inconsistency and the UK courts are wrongly, in my view, not [always] allowing reasonable excuse to be used and failing to give adequate protection to your right to free speech.”

Russenberger, Dames and co-defendants Cosmo Cattell, Jane Touil, Clara O’Callaghan and Michael Dunk took part in the M25 protest, which spanned four days, on 8 November 2022. Rock, Sam Holland and Rachel Payne, who were found not guilty at Guildford crown court, participated on the following day.

Mel Carrington, a JSO spokesperson, said: “[Judges who] deny juries their right to determine whether or not our actions were justified are not protecting the public. They are acting anti-democratically and immorally.”

Ruth Ehrlich, head of policy and campaigns at human rights defence group Liberty, said: “The government must review its anti-protest laws in light of the mounting evidence they aren’t fit for purpose.”

A spokesperson for judges in England and Wales said: “Judicial independence and impartiality are fundamental to the rule of law. In each case, judges make decisions based on the evidence and arguments presented to them and apply the law as it stands.”

The Crown Prosecution Service has been approached for comment.

In March, JSO announced an end to its campaign of civil resistance, its demand to end new oil and gas having become government policy.