Activists gather on the occasion of the global climate strike day, April 19, in Bern, Switzerland. The protest is part of the global climate strike of the international movement to prevent climate change. AP-Yonhap

A climate activist should not be prosecuted for holding a placard outside a criminal court telling jurors they could acquit defendants according to their conscience, a judge in London ruled on Monday.

Retired social worker Trudi Warner, 69, was accused of contempt of court for standing outside London's Inner London Crown Court with the placard last year ahead of a trial of environmental activists from the group Insulate Britain.

Her sign told jurors that "you have an absolute right to acquit a defendant according to your conscience", which Warner says she displayed after a number of activists were barred from mentioning climate change as part of their defence in previous trials.

Britain's Solicitor General, one of the government's law officers, argued that Warner's protest was an attempt to interfere with the administration of justice by inviting jurors to give a particular verdict.

This, he argued at a hearing last week, amounted to contempt of court, a criminal offence which can be punished by up to two years in prison and an unlimited fine.

But Judge Pushpinder Saini dismissed the case on Monday, saying a criminal prosecution was a "disproportionate approach" which unlawfully interfered with Warner's right to freedom of expression.

Warner said after the ruling: "I simply wanted jurors to know their rights and I wanted jurors to be confident enough to be able to stand up to judges who were, in my opinion, acting unlawfully."

Asked if she would repeat her actions, Warner said: "Probably."

The attempted prosecution of Warner comes amid a wider crackdown on protest movements in Britain and across Europe, as environmental activists use direct action protests to demand urgent government action against climate change.

Britain has given police wider powers to restrict peaceful protests, including by imposing conditions if a demonstration could cause "serious disruption to the life of the community", which is currently subject to a legal challenge.

Environmental activists have in some cases defended themselves against charges of causing criminal damage, without disputing having done so, arguing owners would have consented to the damage had they known the full extent of climate change.

Last month, however, that defence was severely curtailed by the Court of Appeal, which ruled activists accused of criminal damage cannot rely on their political or philosophical beliefs.

Warner said outside court that the ruling had left activists with few legal defences, apart from jurors returning verdicts according to their conscience.

"The power rests with us and this is why jurors are a threat to the state," she said. (Reuters)