Britain is to formally present its case to the UN in New York for extending its territorial rights around Ascension Island in the South Atlantic.
States have rights over their resources – including oil or gas reserves – up to 200 nautical miles from the shoreline.
But the UK wants to extend those rights around Ascension on the grounds that the island’s landmass actually reaches much further into the sea underwater.
Ascension Island is part of the British overseas territory of St Helena.
The UK will present its claim on Wednesday to the United Nations Commission for the Limits of the Continental Shelf.
In recent years, battle for control of the world’s seabed has intensified because new technology has made any reserves of oil and gas there much more accessible.
Experts say that fewer than half of the world’s maritime boundaries have been agreed, and there is significant potential for conflict where more than one country submits claims to overlapping areas.
Investigating these claims involves costly, complex and time-consuming sub-sea surveys.
They must verify the ultimate extent of a territory’s landmass – or continental shelf – in order to determine 200 nautical miles from that limit.
In 2006, Britain submitted a joint claim with France, Spain and the Irish Republic for the continental shelf in part of the Bay of Biscay.
Then, in May this year, the bid for Ascension Island was lodged.
There are no other states involved, so the success of the case is likely to rest on the degree to which Britain’s claim is valued scientifically.
Britain is also said to be considering making a similar claim around the Falkland Islands, where it is thought there could be significant reserves of oil and gas.
This would be deeply unpopular with Argentina, which claims ownership of the Falklands, and Argentine officials have told the BBC they are preparing their own counter-claim.
Greenpeace is concerned that what it calls this new colonialism could result in drilling which would harm the fragile ecosystem of the ocean.
Countries are claiming rights to parts of the Antarctic, in case a 1991 UN treaty banned drilling there is ever overturned.
A Foreign Office spokesman has said that no formal submission has been made regarding the British Antarctic Territory, “although we reserve the right to do so”.
Britain is also in discussions with Iceland, Ireland and Denmark – on behalf of the Faroe Islands – about a joint claim in the Hattan-Rockall area of the North East Atlantic, off the west coast of Scotland.
Sea beds beyond the continental shelf are referred to as “The Area” and any world state – landlocked or not – has equal rights.
Last year, Russia planted a flag on the seabed of the North Pole, sparking concerns among other countries, including Canada, that a large-scale land grab was imminent.