The International Court of Justice has ruled that Uruguay can continue building two pulp mills which Argentina argues will pose a pollution threat.
The judges' decision means construction can proceed while they consider the overall case for or against the mills.
The dispute has strained ties between the two normally friendly neighbours.
Argentina says the mills could pollute their border river but Uruguay says they will generate jobs and be under strict environmental control.
Judges at the International Court of Justice in The Hague voted 14-1 in Uruguay's favour.
The Uruguayan ambassador to The Hague, Carlos Mora, said he was very pleased with the outcome.
But the news was greeted in silence by those opposed to the project in the Argentine town of Gualeguaychu, on the other side of the river to the mills.
They had gathered in a local theatre in the early morning to hear the outcome, and vowed to continue their protest.
It is the latest chapter in a long-simmering row that has provoked months of protests in both countries.
A great deal of damage has already been done between two historically friendly governments and peoples, the BBC's South America correspondent Daniel Schweimler says.
The Argentine government wanted construction stopped to allow further environmental studies to be conducted.
Uruguay argued that the mills would adhere to the strictest rules and would bring badly needed jobs to the border area.
The court ruled that the construction of the mills posed no serious threat to the environment and could continue while the judges evaluate the potential risks of the pulp plants once they begin operation.
The circumstances did not require a provisional measure ordering the suspension of the mills' construction, the president of the court, Rosalyn Higgins, said.
The mills are being built on the Uruguayan side of the River Uruguay that separates the two neighbours.
Argentina said that they would pollute a region dependent on agriculture and tourism.
Under a 1975 treaty, all issues concerning the River Uruguay must be agreed by both countries.
Uruguay approved one mill in 2003 and has "aggravated the dispute" by authorising a second one, the Argentine application to the court said.
Uruguay argues that the Spanish and Finnish companies building the mills will use the latest technology to avoid pollution, and the $1.7bn (Â£920m) project will stimulate the local economy.
There have been mass protests in Argentina against the mills.
In April, thousands blocked traffic crossing the bridge between the two countries, while a bikini-clad beauty queen interrupted a recent summit of EU and Latin American leaders in Vienna.
Environmentalists have also demonstrated in Uruguay.
Now the ICJ has issued its provisional ruling, it will move on to study the substance of Argentina's complain, which could take several years.
- 183 Environment