The law in Northern Ireland which has allowed people to be convicted of murder even if they did not inflict the fatal blow has been wrongly interpreted for more than 30 years, the Supreme Court has ruled.
The joint enterprise law has been used to convict people in gang-related cases if defendants “could” have foreseen violent acts by their associates.
However, judges ruled it was wrong to treat “foresight” as a sufficient test.
Their decision could pave the way for hundreds of prisoners to seek appeals.
It will apply in England, Wales, Northern Ireland and most UK overseas common law territories but not in Scotland, which has its own rules on joint enterprise.
Campaigners against joint enterprise welcomed the ruling, saying it would mean a fairer law – but some murder victims’ relatives said they were worried about possible appeals.