LONDON Britain's Supreme Court ruled on Thursday that a key part of the law often called "joint enterprise", which allows people to be convicted of murder even if they did not strike the fatal blow, had been misinterpreted for three decades.
The ruling could lead to a rush of appeals, although the court warned that its decision did not mean that all past convictions in which the specific point of law at issue played a part should be overturned.
The court was ruling on appeals against murder convictions by two men serving life sentences in separate cases. The appeals were considered together because both hinged on the joint enterprise issue.
The crucial question that the five judges had to consider was what is the mental element that has to be proven in order to convict a person of a crime actually committed by someone else.
Since two landmark judgments from 1985 and 1999 that were binding on all judges under Britain's common law system, it was enough to prove that a suspect foresaw the possibility that the main culprit would commit the crime.
But the Supreme Court ruled unanimously that those two judgments had taken a "wrong turn" in law, and that in fact what had to be proven was intent to assist or encourage the main culprit to commit the crime.
The judges cautioned that in many cases, the result may well have been the same, since foresight could potentially be used as evidence of intent.
The point of law does not only apply to murder but can also arise in cases of fraud or other criminal ventures. However, its most controversial uses over the years have been in murder cases.
The Supreme Court set aside the two murder convictions that were being appealed, but did not quash them. There will be further legal argument on whether there should be re-trials or whether the convictions should be converted to manslaughter.
The first case concerned a man called Ameen Jogee, who was convicted of murder over the fatal stabbing in 2011 of Paul Fyfe by another man, Mohammed Hirsi. Jogee and Hirsi had spent the previous hours together drinking and taking drugs, and both men were involved in an angry confrontation with Fyfe.
At one point, Jogee threatened to smash a bottle over Fyfe's head, but it was Hirsi who killed him with a kitchen knife.
Jogee's lawyers said the joint enterprise law, as applied before Thursday's ruling, had "over-criminalised" secondary parties and they were glad to have played a part in correcting "this unjust law".
(Reporting by Estelle Shirbon; editing by Guy Faulconbridge)