Britain seeks to expand its empire with 77,000 square miles of Atlantic seabed

Britain has made a claim to extend its territorial boundary around Ascension Island in the South Atlantic to give it exclusive rights over any natural resources that may be found in the sea or on the seabed up to 350 miles from the island.

At present, Britain can claim exclusive rights up to 200 miles from the shoreline of Ascension Island but under an international convention this can be extended a further 100 miles if a country can show that the continental shelf extends well out to sea.

If the United Nations Commission on the Limits of the Continental Shelf – which is in session in New York – rules in favour of Britain's submission, it will mean an additional 77,226 square miles of seabed in the South Atlantic will come under British control.

The plan to extend exclusive rights around Ascension, which is the peak of a 13,000ft mountain in the middle of the ocean, could be followed by a British attempt to extend territorial claims around the Falklands, which would almost certainly be opposed by Argentina.

No formal submission has yet been made for the continental shelf around the Falklands, but geological experts believe it is only a matter of time, given that there is likely to be rich deposits of oil and gas in the 350-mile limit around the islands. It is unlikely that the area of the seabed around Ascension being claimed by Britain will hold any major oil or gas fields but it could contain rich mineral deposits.

Ascension Island is part of the British overseas territory of Saint Helena, some 750 miles south-east. Executive authority is vested in the Queen, who is represented on Ascension by the administrator. Britain claims that Ascension Island has a long history of occupation and economic activity that qualifies it under the terms of the international law of the sea as both an exclusive economic zone and as a continental shelf.

In its submission to the UN commission, Britain says that the area of the continental shelf around Ascension is not subject to any dispute with other nations, which means that the judgment will focus purely on the scientific case for extending British rights based on defining the boundary of the continental shelf.

Martin Pratt, director of Durham University's International Boundaries Research Unit, said: "States with broad continental margins also have rights to exploit the resources of the continental shelf beyond 200 nautical miles, but they are required to provide the commission with scientific evidence concerning the location of the outer limit of the shelf."

Like many countries, Britain is making sure its claim to extend territorial influence is made before the deadline ends next year. "If states don't claim it, the seabed becomes part of what is known as 'the area', which comes under international control," Dr Pratt added. "It's really a 'claim it or lose it' scenario."

And countries may also claim the seabed off Antarctica, where a special treaty is aimed at preserving the unique landscape. Britain has not made a formal submission but has said that it reserves the right to do so given the interest of other nations with scientific bases in the Antarctic.

In the Arctic, half a dozen countries are likely to make competing claims to exclusive exploitation of the seabed. The most controversial is that made by Russia, which is claiming that the Lomonosov ridge on the Arctic seabed is part of its continental shelf.