Britain's relationship with France was strained at the height of the Falklands War over fears the French could allow Argentina to acquire Exocet missiles, previously secret files showed on Friday.

Publicly the then prime minister Margaret Thatcher praised president Francois Mitterrand for his support during the conflict, but papers released by the National Archives reveal London's intense suspicion of the French.

At one point, a furious Thatcher warned Mitterrand it could have "disastrous" consequences for the entire NATO alliance if a fresh delivery of the French-built Exocets was allowed to reach Argentina.

Britain saw the threat of the Exocet on May 4, 1982, when a pair of Argentinian air force fighters attacked the British naval task force heading to the Falklands and fired two sea-skimming guided missiles.

One Exocet hit the Type-42 destroyer, HMS Sheffield, crippling the ship -- which eventually sank six days later -- and killing 20 crew.

It was thought the French had supplied around 100 Exocet AM39s to air forces around the world, of which 30 to 35 could be available for sale on the international market.

Such were the fears that Britain's foreign intelligence agency MI6 launched a "James Bond-style" deception operation designed to convince the Argentines they were buying Exocets on their behalf, when the real aim was to ensure no missiles ever reached Argentina.

In London, concern was growing that France was preparing to release a consignment of four Exocets to Peru, despite clear warnings from British and French intelligence that they would end up in Argentina.

Thatcher raised the issue with Mitterrand directly and won an assurance that the missiles would be delayed "as long as necessary".

Two weeks later on May 29, Mitterrand telephoned Thatcher to say he was in a "difficult position" over the Peruvian deal because it was putting France's contracts with other Latin American countries "in danger", the files show.

When the president asked for a "precise estimate" of the date by which the missiles would no longer represent a threat to British forces heading for the Falklands, Thatcher was dismayed.

The following day she fired off a blistering telegram.

"If it became known, as it certainly would, that France was now releasing weapons to Peru that would certainly be passed on to Argentina for use against us, France's ally, this would have a devastating effect on the relationship between our two countries," she told Mitterrand.

"Indeed, it would have a disastrous effect on the (NATO) alliance as a whole. This is the last thing that either of us would wish. I greatly hope therefore that for the time being you will be able to find some way of keeping these missiles in France."

Her protests had the desired effect -- France told the Peruvians that the missiles could not be sent for "political reasons".

Other previously secret papers reveal Thatcher was "taken by surprise" by the Argentine invasion.

She dispatched a British naval task force which retook the South Atlantic islands after a short war that left 649 Argentines and 255 British soldiers dead.