Mr Justice Saini said there had been no basis for the prosecution of Trudi Warner, 69, for criminal contempt for holding a placard outside the trial of climate activists that told jurors they had the right to acquit based on their conscience.

The judge said that government lawyers had mischaracterised the evidence when they said that Ms Warner had acted in an intimidating and abusive manner.

The attorney general pursued Ms Warner, a retired social worker, for contempt of court after her protest outside Inner London Crown Court.

In yesterday’s ruling, Mr Justice Saini said: “The solicitor general’s case does not disclose a reasonable basis for committal … the conduct did not amount to an act of contempt. I refuse the solicitor general permission to proceed, and I dismiss the claim.

“It is fanciful to suggest that Ms Warner’s behaviour falls into the category of contempt. The category is limited to threatening, intimidatory, abusive conduct or other forms of harassment.”

Ms Warner’s sign was a reference to the 1670 case that established the independence of juries, “Bushel’s case”.

Her placard read: “Jurors, you have an absolute right to acquit a defendant according to your conscience.”

Ms Warner said: “What I was doing was drawing attention to the terrible repression of conscientious protectors, and in particular climate protesters, by the state. If what I did will empower other defendants to use the power to acquit by juries, this will have been the fight of my life.”