Britain repelled an Argentine invasion of the British-owned islands in 1982 - now the UN extends Argentina's territorial waters.
Argentina is celebrating a UN commission decision that has increased its maritime territory in the South Atlantic Ocean to include the disputed Falkland Islands and beyond.
Argentina's foreign ministry said its waters had been increased by 0.66 million square miles (1.7 million square kilometres) - 35% - and the decision will be key in its dispute with Britain over the islands which it calls Las Malvinas.
"This is a historic occasion for Argentina because we've made a huge leap in the demarcation of the exterior limit of our continental shelf," foreign minister Susana Malcorra said.
"This reaffirms our sovereignty rights over the resources of our continental shelf."
The islands were at the centre of a dispute in 1982 when Argentina seized the South Atlantic archipelago and put its troops on British sovereign territory.
Britain repelled the occupation and the islands are now self-governing, with Britain responsible for its defence and foreign affairs - and many islanders still want no connection with Argentina.
The UK says the islanders cannot be forced to accept Argentine rule against their will.
Earlier this month, the UN Commission on the Limits of the Continental Shelf sided with Argentina ratifying the country's 2009 report fixing the limit of its territory at 200 to 350 miles from its coast.
However, the UN did note that the diplomatic dispute between Argentina and Britain remained unresolved.
The Falkland Islands government has said it is looking to Britain to discover "what, if any, decisions have been made, and what implications there may be" now for the territory.
"Our understanding has always been that the UN would not make any determination on applications for continental shelf extension in areas where there are competing claims," said spokesman Mike Summers.
The Falklands' economy is benefiting currently from the millions of dollars being spent on oil exploration in the region.
The British Government has not yet commented.
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