Uruguay's Supreme Court declared unconstitutional a law that provided amnesty to military officials accused of murders, disappearances and other human rights violations during the country's dictatorship.
Ruling in the case of a young communist detained and killed by the military in 1974, the justices said the amnesty law violates Uruguay's separation of powers and failed to pass by a required supermajority - arguments sure to be made in other dirty war prosecutions.
The law remains on the books, but the ruling could swing voters in favour of overturning it altogether in a plebiscite being held on Sunday along with presidential elections. Until now, polls have shown the plebiscite failing.
Former vice president and constitutional scholar Gonzalo Aguirre said the ruling "supports the conviction that on Sunday the law should be annulled by popular vote and that this will lead to the reopening of dozens of cases that could not be prosecuted because of the amnesty".
Court spokesman Raul Oxandabarat said he can't release details of the 4-1 ruling because it hasn't yet been delivered to all the parties in the case.
But lawyer Jose Errandonea, who represents the family of the murdered militant Nibia Sabalsagaray, said "the ruling is so convincing in its fundamentals, and while each case is unique, it's understood that this sets a precedent, and that the Supreme Court won't change if presented with a similar case,"
Amnesties for human rights violators were key to enabling democracies to emerge from the dictatorship era in South America, but they have been increasingly challenged recently.
Uruguay's law granting amnesty to military figures in the 1973-1985 dictatorship was passed by a congressional majority in 1986 and reaffirmed in 1989 with 54% of the vote in a plebiscite.
It was seen initially as a balancing response to a 1985 law granting amnesty to leftist guerrillas accused of attacks.