In a London court in 1670, a judge, livid with the jury, locked them away for two days without food, water or even a chamber pot. The jury’s offence? Defying the judge’s direction to convict the Quaker William Penn – the future founder of Pennsylvania – charged with preaching sedition in the City of London. The foreman, Edward Bushell, would not yield and, when the matter reached the chief justice of England, he ruled that no juror could be punished for their refusal to convict, entitling a jury to decide according to its conscience, whatever the bench directed. A plaque honours Bushell at the Old Bailey, so jurors on their way inside may contemplate the man who secured their right to acquit.

The legal principle has held for three and a half centuries and, in my 50 years of practice, I have witnessed many juries bring back “sympathy verdicts”, that is, acquittals, because they think a defendant has been oppressively or unfairly prosecuted. But they are not usually reminded by barristers of their right to do so because of the profession’s concern that they should not be urging juries to lay aside the oath they took to decide according to the evidence.

This right sits at the centre of the case of the Elbit four who last week were sentenced to more than 22 years combined, for their part in a direct action protest. Leona Kamio, 30, a nursery teacher, Samuel Corner, 23, and Fatema Rajwani, 21, both students, and Charlotte Head, 30, a domestic abuse caseworker, broke into a factory owned by Elbit Systems, an Israeli company that manufactures drones. They are among more than two dozen people – “the Filton 25” – being tried for breaking into an arms factory in Filton near Bristol, or in connection with the act. They are now in the process of going to court, spread over four trials.

The first group of defendants underwent not one, but two trials. At their first, they faced several charges, the gravest being aggravated burglary. Their leading counsel, Rajiv Menon KC, took to the floor to remind the jury of their historical right to acquit, and invited them to weigh the use to which the drones were being put. The judge, Mr Justice Johnson, referred Menon to the high court to be tried for the crime of contempt of court for breaching his order not to mention the jury’s right to acquit. The jury, however, went on to acquit the defendants of aggravated burglary. But they could not come to a majority decision on any of the remaining charges.

The prosecution decided that the defendants must face a retrial. This proceeded with the same Mr Justice Johnson presiding on the charges the first jury could not resolve: criminal damage, an offence ordinarily met with a fine or a sentence of 18 months or so. At this point, the four had already spent more than 18 months in pretrial detention. The second jury convicted the Elbit four of criminal damage.

One defendant was also convicted of grievous bodily harm without intent, having injured a female police officer with a sledgehammer. He said, and the jury must have accepted that, disoriented by the Pava pepper spray the officer had just deployed, he had swung the hammer to shield a co-defendant, accidentally hitting the police officer. His sentence nonetheless was seven years and eight months.

How does damaging property earn nearly eight years? At a secret hearing during the first trial, Mr Justice Johnson had ruled that the protest carried a “terrorist connection”. This was unprecedented in the history of direct action trials. His reasoning was that the defendants’ purpose was to “influence” the British government – which is the purpose of almost every political protest ever mounted. The judge’s ruling was kept secret from both the jury and the public, and the UK press was barred from reporting on it.

There was no terrorism in any ordinary sense: no violence meant to kill or maim, only a determination to expose British complicity in the killing of Palestinians. But the Sentencing Act of 2020, passed by the then Conservative government, has significantly widened what counts as terrorism. That fateful act allowed the judge to impose a far heavier sentence. As “terrorists”, they will serve longer times in prison before they are up for parole and 15 years on a list that makes them police suspects for real terrorism crimes.

The Elbit four will be labelled as “terrorists” because they were convicted, in substance, of a quasi-terrorist offence that was never charged, never put to the jury, and never proven by the prosecution. The jurors who found them guilty of criminal damage had no idea their verdict would be treated as a verdict on terrorism. The prosecution was not required to establish the terrorist connection beyond reasonable doubt, or to any standard at all.

It is another foundational principle of English law that no one should be convicted of an offence that has not been charged and proved. In this case, the principle was suspended. The secrecy compounds the injury. The open-justice principle exists, as Jeremy Bentham put it, because “publicity is the very soul of justice; it keeps the judge, while trying, under trial”.

The court of appeal struck down the trial judge’s decision to have Menon tried by the high court for contempt. The better view is that contempt citations should be that contempt citations should be referred to the ttorney general to decide whether the public interest justifies a prosecution. The court ruled that Johnson’s decision was wrong; he apologised, but that did not stop him sentencing Menon’s clients when they were convicted at the second trial.

All of which returns us to Edward Bushell, and to what a jury is for. Juries have always had the power to temper law with mercy. It is among the oldest protections against an overbearing state. The difficulty is that judges seldom tell juries the power exists, leaving counsel to invoke it only obliquely.

Years ago, defending a woman who had killed a man for beating her every day of their life together, the great advocate Edward Marshall Hall closed with the words: “Just look at her, gentlemen of the jury. God never gave her a chance. Won’t you?” They did.

It would be far more transparent to bring the power into the open – to let the judge, where the defence claims it, remind the jury of the right, and let the prosecution argue against its exercise. Instead, the only lawful way to appeal to a jury’s conscience is to tell them to look at a plaque in the Old Bailey lobby.

The Elbit four did not act in ignorance of the consequences. Bentham held that a citizen may disobey a law they believe to be unjust, provided they are willing to accept the punishment. On that view, the protesters who knew very well they were breaking the law against criminal damage should have pleaded guilty. While on trial, they openly acknowledged participating in the factory break-in and damaging the drones. What no defendant should have to face is punishment for a crime of terrorism that was never charged.

• A version of this article was originally published in The Key magazine

• Geoffrey Robertson KC is founding head of Doughty Street Chambers and his latest book is World of War Crimes – Eyeless in Gaza and Beyond