Britain would never be able to recapture the Falklands if it lost control of the island's military base, a leading military think-tank has said.

In an analysis of the defence situation in the South Atlantic published today, professor Michael Clarke, the director-general of the Royal United Services Institute (RUSI), said control of the Mount Pleasant airbase was crucial to the defence of the islands.

"The military balance around the Falklands has been transformed by the construction since 1982 of the Mount Pleasant airbase 30 miles outside Stanley, which replaced the small airstrip that existed at that time," he said.

"Mount Pleasant is a major military facility with two runways of over 4,000 meters between them. It will take military aircraft of all types and as such is a critical military asset that transforms the 'can we defend/re-take The Falklands?' equation into a simple military fact: whoever controls Mount Pleasant controls the Islands.

He added: "As long as Britain occupies the base competently, Argentina could never mount a successful invasion; if Britain ever lost Mount Pleasant to a competent occupier, its forces would never get back onto the Islands, even with twice the military assets now available."

The Royal Air Force currentl has four new Typhoon jets stationed at the base, which Clarke said were the most advanced aircraft in the region by a long way.

By comparison the Argentinean air force is still stocked with French made Mirage jets that it used during the 1982 war with Britain.

In his report Clarke concludes that any Argentinian invasion force would have to cross 400 miles of open sea and air space to get within striking distance, therefore dooming any attack to failure.

"All the advantages lie with the defenders of Mount Pleasant and 24 hours notice of attack - or even something that looks like preliminary preparations for an attack - is all that would be required to make it impregnable as long as London had the will to act," he said.

Argentina has threatened to take legal action against British companies involved in oil development in the Falkland Islands - a move described by the Foreign Office as "wholly counter-productive".

Tensions between Britain and Argentina over the sovereignty of the Falklands have risen in recent weeks as the 30th anniversary of the 1982 war approaches.