For decades debate and recrimination has raged over where the ship was heading when it was torpedoed by a Royal Navy submarine.
Britain received international criticism after the sinking after the Argentine Junta announced that the warship had been returning to its home port and was outside the 200 mile exclusion zone imposed by Whitehall.
But Major David Thorp, who spent 34 years working as a signals expert in military intelligence, has disclosed for the first time that he was asked to carry out a trawl of all the intelligence on the sinking at the direct request of Margaret Thatcher a few months after the end of the war.
He was ordered to compile a report for the Prime Minister called “The Sinking of the Belgrano” that has never been published.
From his own signals intercepts and those from other Government agencies, he proved that the Argentine cruiser was heading into the exclusion zone.
Major Thorp was in charge of a top secret signals interception section hidden on the amphibious warship Intrepid as it steamed with the Task Force.
Around Ascension Island, 4,000 miles from the Falklands, his team began picking up naval communications sent to the Argentine fleet which they were easily able to decipher.
The report states that in late April 1982, they intercepted a message sent from naval headquarters ordering the Belgrano and its escorts to a grid reference within the exclusion zone and not back to base as the Argentines later claimed.
The Belgrano was sunk by two torpedoes fired by the hunter-killer submarine Conqueror on May 2 with the loss of 323 lives a number of miles outside the exclusion zone.
“For some reason they decided on a rendezvous point still within the exclusion zone,” Major Thorp said. “Whether they were trying to raise a thumb at us I don’t know. At the time I thought it was strange thinking why didn’t they go straight into port?”
In his new book, The Silent Listener, Major Thorp wrote: “The findings of my final report stated the destination of the vessel was not to her home port as the Argentine Junta stated but the objective of the ship was to relocate to a prearranged RV within the exclusion zone.”
Despite the report being read by Mrs Thatcher she never disclosed the information either in Parliament or elsewhere possibly because she did not want to reveal Britain’s eavesdropping capabilities.
But during her infamous BBC exchange with the schoolteacher Diana Gould who confronted her on the sinking Mrs Thatcher made an intriguing reference to the report saying: "One day, all of the facts, in about 30 years time, will be published." Mrs Gould died earlier this month.
In recent years the Argentine navy has accepted that the sinking of the Belgrano was a legitimate act of war.
In his book, that was cleared by the security services, Major Thorp also discloses for the first time how the British code-cracking operation gave the force a significant advantage.
Shortly before the Battle of Goose Green, Lt Col “H” Jones, the commander of 2nd Bn The Parachute Regiment, boarded Intrepid after hearing about the eavesdroppers through SAS colleagues.
“That morning we had picked up 10 grid references on intercepts and H looked at the map and realised that they were his own troops’ locations. He said “bloody hell we are sharing the same hill as the enemy.’”
“He wanted to know the strengths and weaknesses of the Argentines, then we looked at calibre of people on ground and he came to the conclusion that perhaps 600 Paras were worth 1,500 Argentines.”
The intelligence gave the commanding officer the “peace of mind” to start the battle that would lead in his own death, a posthumous Victoria Cross award and ultimately victory in the campaign.
* Notícia publicada pel Telegraph. Creiem que, malgrat el temps passat, és bó que es conegui la veritat sobre l'enfonsament del Belgrano, que fou un acte de guerra perfectament legítim.