VITAL intelligence about the most controversial action of the Falklands campaign in 1982 did not reach senior British commanders and politicians until it was too late, the official history of the conflict, which is published tomorrow, discloses.

But there were few regrets that the British nuclear-powered submarine HMS Conqueror sank the Argentine cruiser General Belgrano with the loss of 321 lives, even though the enemy had changed its mind about launching an attack on the British task force and was travelling away from the battle zone.

Sir Lawrence Freedman, the historian who has compiled an exhaustive three-volume account of the war, also tells for the first time of an SAS mission to sabotage an Argentine bomber base, which was foiled by bad weather, and of the nuclear depth charges routinely carried by four Royal Naval ships in the South Atlantic.

Although widely lauded at the time for her principled stand against the Argentine invasion of the islands, Margaret Thatcher endured years of criticism for allowing the attack on a warship outside the British-imposed exclusion zone around the islands. It now appears that she did not know the full story.

On May 1, 1982, Admiral Juan Lombardo ordered all Argentine naval units to seek out the British task force around the Falklands and launch a “massive attack” the following day. The Belgrano, which was outside the exclusion zone to the north, was ordered south.

Lombardo’s signal was intercepted by British Intelligence. As a result Mrs Thatcher and her War Cabinet, meeting at Chequers the following day, agreed to a request from Admiral Sir Terence Lewin, the Chief of the Defence Staff, to alter the rules of engagement and allow an attack on the Belgrano outside the exclusion zone.

It was a controversial decision which led to allegations that the enemy cruiser had been attacked to scupper a late peace plan proposed by the President of Peru.

Other intercepted signals told the British that the Argentinians had been ordered to turn back at the moment and resume their previous positions, because they had been spotted by a Royal Navy Sea Harrier. But the contents of these later signals “were not distributed on the British side until the next day and had no influence on the day’s events”, the official history says.

Commander Christopher Wreford-Brown, the commanding officer of HMS Conqueror, had sent a signal four hours before he fired his torpedoes at the Belgrano, saying that the Argentine cruiser had changed course.

His signal was received in Britain by Vice-Admiral Peter Herbert, Flag Officer Submarines, but it was not passed on to the MoD or to Rear-Admiral Sandy Woodward, who was commander of the task force in the South Atlantic.

Professor Freedman says that Admiral Herbert believed the task force “had to take its chances when it could”, and that, even if he could have gained the political authority to rescind the new rules of engagement (RoE), it did not seem sensible.

“The change in the RoE was seen as a necessary step that would have to be taken at some point, to enable the task force to engage an enemy that was clearly geared up for battle,” the author says.

Professor Freedman also relates how the War Cabinet approved a move to protect Royal Naval ships from air attacks by blowing up an airbase full of Super Etendard bombers equipped with Exocet missiles.

Admiral Lewin proposed that special forces should undertake an intelligence-gathering mission, followed by an attack on the Super Etendards at the Rio Grande airbase in Tierra del Fuego by 55 SAS men.

Officially codenamed Operation Mikado, the SAS knew it as Operation Certain Death.

A reconnaissance helicopter took off on May 17 but bad weather forced it to land 50 miles from its target, and the mission was aborted. The crew set fire to it to make it look as though it had crashed, and crossed the border into Chile.

Before the main part of the mission could be launched, the enemy had surrendered.

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