A law shielding officials of the last military government from prosecution for abuses is unconstitutional, the Uruguayan Supreme Court has ruled.
The court's ruling came in the case of a young Communist opponent of the military government who was allegedly tortured and killed in 1974.
A referendum on Sunday, held alongside the presidential election, will ask voters if they want to scrap the law.
Some 200 Uruguayans disappeared during military rule from 1973 to 1985.
The Supreme Court's ruling came in the case of Nibia Sabalsagaray. She died 35 years ago in a military barracks.
A lawyer representing her family told the Associated Press that while each case was unique, "it's understood that this sets a precedent and that the Supreme Court won't change if presented with a similar case".
On Sunday, Uruguayan voters will decide whether the amnesty law should be overturned.
So far opinion polls have suggested the measure will fail to win the 50% support required, but the court's ruling could be significant.
"If the law is annulled, there won't be any sense in ruling that it's unconstitutional, but if the referendum doesn't get enough votes, there is the possibility of making this argument in other cases," prosecutor Mirtha Guianze, who brought the Sabalsagaray case to the Supreme Court, told Reuters news agency.
Similar amnesty laws were passed in neighbouring South American countries, including Argentina, Brazil and Chile as military rule from the 1970s and 1980s came to an end.
Argentina has since scrapped its legislation.