Prime minister Margaret Thatcher was warned about the risks in slashing Britain's navy, a year before the 1982 Argentine invasion of the Falkland Islands, secret files released Friday showed.

Her foreign secretary Lord Peter Carrington also warned defence secretary John Nott that axing Britain's Antarctic patrol ship would send all the wrong signals about London's willingness to defend the Falklands.

Documents released after 30 years locked away in Britain's National Archives showed that the head of the Royal Navy was fuming in 1981 about planned defence cuts.

First sea lord Admiral Henry Leach, who later told Thatcher that Britain could and should send a naval task force to retake the Falklands following the April 1982 invasion, was furious with her a year earlier over her "unbalanced devastation" of the armed forces.

"I note with regret but understanding that the tightness of your programme precludes your seeing me personally as requested," he wrote in a stinging note to the premier.

"I am confident however that you will at least spare two minutes to read this note from the professional head of the navy before you and your Cabinet colleagues consider a proposition substantially to dismantle that navy."

Leach concluded: "We are on the brink of a historic decision.

"War seldom takes the expected form and a strong maritime capability provides flexibility for the unforeseen. If you erode it to the extent envisaged I believe you will foreclose your future options and prejudice our national security."

The files also include a letter from Carrington to Nott, urging him against axing the Antarctic patrol ship HMS Endurance, warning it would send the wrong signal to Buenos Aires at a time of tension over the Falklands.

"Unless and until the dispute is settled, it will be important to maintain our normal presence in the area at the current level," he wrote.

"Any reduction would be interpreted by both the islanders and the Argentines as a reduction in our commitment to the Islands and in our willingness to defend them."

His appeal was, however, rejected and HMS Endurance was less than a month from being withdrawn from service when the first Argentinians landed on Britain's South Georgia overseas territory in March 1982.

Diplomatic friction between Argentina and Britain has intensified since 2010, when London authorised oil prospecting around the islands, which have a population of around 3,000.

Britain would "never" negotiate the sovereignty of the Falklands against its citizens' wishes, Prime Minister David Cameron said in a Christmas message to the Falklands.

Thatcher, now 86, retired from politics and suffering from dementia, is back in the limelight thanks to the biopic film "The Iron Lady", which hits cinemas around the world next month.