A South American trading bloc has agreed to close its ports to ships flying the Falkland Islands flag.
Mercosur, which includes Brazil, Argentina, Paraguay and Uruguay, came to the decision at a summit in the Uruguayan capital, Montevideo.
But Uruguayan President Jose Mujica also stated his country would never impose a maritime blockade of the British overseas territory.
The UK has asked Uruguay, which proposed the move, to explain itself.
The Mercosur decision is the latest in a series by Latin American regional bodies designed to show solidarity with Argentina, which has long claimed sovereignty over the Falkland Islands, which it calls Las Malvinas.
Britain has held them since the 1830s and says their future is not negotiable. The two countries fought a brief but bloody war over the islands in 1982.
The Falklands flag is flown by 25 boats, mostly fishing vessels.
Mr Mujica said: "We hold nothing against the UK. But we have a lot in favour of Argentina."
However, the president of the Falklands Chamber of Commerce, Roger Spink, told the BBC that they were a small community, and felt increasingly under blockade.
"If we were Palestine, the European Union would be up in arms," he said.
Last week the Foreign Office called on Uruguay's ambassador in London to explain the move.
Argentina's President Cristina Fernandez de Kirchner, who took over the presidency of the trade bloc from Mr Mujica, thanked her fellow presidents for the show of support.
Delivering a speech to the summit, she said: "I want to thank everyone for their immense solidarity with the Malvinas.
"But you should know that when you are signing something on the Malvinas in favour of Argentina you are also doing it in your own defence.
"Malvinas is not an Argentine cause, it is a global cause, because in the Malvinas they are taking our oil and fishing resources.
"And when there is need for more resources, those who are strong are going to look for them wherever and however they can."
British companies are exploring for oil in the waters surrounding the islands, which are 400 nautical miles from the Argentine coast.