The law on joint enterprise, which can result in people being convicted of assault or murder even if they did not strike the blow, has been wrongly interpreted by criminal trial judges over the past 30 years, the Supreme Court has ruled.
Video report by ITV News Correspondent Juliet Bremner
The judgment could lead to hundreds of people convicted of joint enterprise crimes launching appeals, including two men convicted of Stephen Lawrence's racially motivated murder in 1993.
Gary Dobson and David Norris were convicted in 2012 for the 18-year-old's murder.
The Supreme Court justices said prosecutors, judges and jurors must take a different approach when dealing with such defendants.
They said it was not right that someone should be guilty merely because they foresaw that a co-accused might commit a crime.
Jurors should view "foresight" only as evidence to be taken into account, not as proof, they added.
The panel analysed the issue of joint enterprise at a hearing in London in October when considering an appeal by a man who was convicted of murder after encouraging his friend to stab a former policeman.
Ameen Jogee was given a life sentence alongside Mohammed Hirsi in March 2012, after encouraging his friend to stab a former police officer.
Hirsi had stabbed Paul Fyfe at a house in Leicester in June 2011 while being egged on by Jogee, the court heard at their trial.
Jogee was convicted of Mr Fyfe's murder,even though he was outside the house when Hirsi stabbed the father-of-three.
A judge imposed a minimum 22-year term on Hirsi, and a minimum of 20 years on Jogee which was later cut to 18 years by the Court of Appeal.
The Supreme Court has now allowed Jogee's appeal against conviction but he will stay in prison while lawyers decide whether he should be retried.
Speaking to ITV News Central after the ruling, Mr Fyfe's widow, Tracey, said: "We thought that would be it today, we finally get to move on.
"He is guilty. I think its an unfair judgement. When they read out the facts it was clear that the joint enterprise law was exactly what had happened."
Mr Fyfe's daughter, Emma Fyfe, added: "We've just got to keep going, keep fighting. We're not going to stop till he's kept inside, that's all we can do really."
Meanwhile, Ameen Jogee's mother Rachael Whitehead told ITV News she was "absolutely overwhelmed" at the court's decision.
She also spoke of her hopes that her son's case will not go to a retrial and that he would soon be freed.
"It's been years of fighting....it's been a rollercoaster ride. It's not over yet as he's not home", she said.