Britain is preparing territorial claims on tens of thousands of square miles of the Atlantic Ocean floor around the Falklands, Ascension Island and Rockall in the hope of annexing potentially lucrative gas, mineral and oil fields, the Guardian has learned.

The UK claims, to be lodged at the UN Commission on the Limits of the Continental Shelf, exploit a novel legal approach that is transforming the international politics of underwater prospecting.

Britain is accelerating its process of submitting applications to the UN - which is fraught with diplomatic sensitivities, not least with Argentina - before an international deadline for registering interests.

Relying on detailed geological and geophysical surveys by scientists and hydrographers, any state can delineate a new "continental shelf outer limit" that can extend up to 350 miles from its shoreline. Data has been collected for most of Britain's submissions and Chris Carleton, head of the law of the sea division at the UK Hydrographic Office and an international expert on the process, said preliminary talks on Rockall are being held in Reykjavik, Iceland, next week.

Mr Carleton believes the Falklands claim has the most potential for acrimonious political fallout. Britain and Argentina fought over the islands 25 years ago, and the value of the oil under the sea in the region is understood to be immense: seismic tests suggest there could be about 60bn barrels under the ocean floor.

Britain has been granted licences for exploratory drilling around the islands within the normal 200-mile exploration limit and any new claim to UNCLCS would extend territorial rights further into the Atlantic.

"It would be beyond the 200-mile limit but less than 350 miles," said Mr Carleton, who is involved in preparing the submission. "It effectively joins up the area around South Georgia to the Falklands. It's a claim but how it's handled has not been decided yet. The Argentinians will say it's not ours to claim. It's all a bit tricky."

Martin Pratt, director of research at Durham University's international boundaries research unit, added: "The Russians may be claiming the Arctic but the UK is claiming a large chunk of the Atlantic. Some states might ask why a big power is entitled to huge stretches of the ocean's resources thousands of miles away from its land, but that's the way the law is."

Because of the sensitivities - earlier this year Buenos Aires scrapped a 1995 agreement with the UK to share any oil found in the adjacent waters - the first formal application from the UK is likely to centre on Ascension.

The volcanic island, 1,000 miles from the African mainland, sits just to one side of the mid-Atlantic ridge. No gas or oil is likely to be found below the surrounding waters but there could be significant mineral deposits on the ocean floor.

Talks have already begun between Ireland, Iceland and Denmark for the division of rights far out into the north Atlantic. It includes the island of Rockall and the sub-sea Hatton ridge. The competing claims are nowhere near final resolution although Ireland and the UK have agreed a common boundary.

Other countries that have submitted claims to the ocean floors around remote overseas dependencies have run into fierce resentment from neighbouring nations. France, which this summer registered its claim to thousands of square miles around New Caledonia, in the Pacific, has received protests from Vanuatu warning that the claim has "serious implications and ramifications on Vanuatu's legal and traditional sovereignty". Russia was criticised this summer for making claims beneath the Arctic Ocean.

The UN body has been progressing slowly through its casework. The process of extending the normal 200-mile limit requires volumes of technical evidence of submarine soundings. According to the convention on the law of the sea, applicant states may register their rights by "establishing the foot of the continental slope, by meeting the requirements stated for the thickness of sedimentary rocks".

Once demarcated, the ocean floor may then be claimed up to 60 nautical miles from the bottom of the continental slope. When territorial rights have been obtained, states have the right to extract any minerals, natural gas or oil discovered in the annexed seabed.

There is a deadline of May 2009 for claims from the UK and other countries to be submitted, although states that ratified the treaty later have more time. "The amount of technical data required is massive," said Mr Pratt. "Australia recently submitted 80 volumes."

In the past, Greenpeace has described the process as a "land grab".

· This article was amended on Friday September 28 2007. About 60bn barrels of oil are estimated to be lying under the seabed around the Falkland Islands, not 60m, as we said in the article. This has been corrected.