The weather conditions were worsening, the sea was becoming more choppy and a light mist had reduced visibility to little more than a mile. The submarine had been trailing the cruiser for more than two hours, coming up to periscope depth four or five times for a visual sighting.

The submarine, the Conqueror, had set out from Rosyth on April 10 on the long journey and it was now three weeks on. The captain of the nuclear-powered sub radioed for permission to attack.

The defence secretary, following Cabinet agreement, later recalled that it was the easiest decision he had to make.

At 18:13, on May 2, the torpedo tubes were loaded with three unguided Mark VIII torpedoes with impact fuses and and 805-pound warheads. These missiles pre-dated the Second World War but were reliable for the close-range attack, unlike with the latest wire-guided Mark 24 Tigerfish torpedoes, which captain and crew found unreliable.

At three minutes before 7pm the captain ordered the firing of the torpedoes.

The first stuck the target near the bow, essentially blowing it off. The second detonated in the machine room and tore through the crew’s nearby mess area which was full of sailors changing shift. The blast killed 275 of its 1,093 crew.

The side-effect, if it can be so described, destroyed the ship’s electricity systems, knocking out the pumps and making it impossible to send out a distress call. The third torpedo missed entirely, struck a nearby ship but failed to explode.

The cruiser which was now listing badly to port was a 44-year-old American-built warship that had survived the Japanese attack on Pearl Harbor.

In 1951, it had been sold to Argentina and renamed the ARA General Belgrano. It would not see out 1982 and survive the Falklands War.

When he was asked later about the sinking of the Belgrano, the Conqueror’s captain, Chris Wreford-Brown, responded wryly: “The Royal Navy spent 13 years preparing me for such an occasion. It would have been regarded as extremely dreary if I had fouled it up.”

The Sun newspaper greeted the sinking of the Belgrano with surely the most infamously memorable headline of its controversial life – GOTCHA – although the text claimed the sinking of another Argentine ship.

The Belgrano had been sunk outside the 200-mile maritime exclusion zone that Britain had imposed around the Falklands and, according to reports, had actually been heading for port.

There had been a Peruvian peace proposal 14 hours earlier, which Britain would later accept (although Argentina rejected it) and in a live television interview on BBC1’s Nationwide the Prime Minister, Margaret Thatcher, denied that she had received it. The Belgrano was a threat to British ships, she said, justifying the action.

In the Green Room after the show, Thatcher’s husband Denis, drink in hand, lashed out at the producer saying that his wife had been “stitched up by bloody BBC p**fs and Trots”.

The controversy over the sinking did not end with the war (although officially it wasn’t one, as there had been no declaration of it).

And it simmered in the most unlikely place, inside the Civil Service in the Ministry of Defence.

At the time, Clive Ponting was a 38-year-old senior official, an assistant secretary, earning £23,000 a year, more than £80,000 today. He was in charge of the MoD’s Defence Secretariat 5 and was heading for the very top.

Thatcher had told the House of Commons the Belgrano was steaming towards the 200-mile exclusion zone imposed by Britain around the Falklands when she was torpedoed, with the loss of 323 of the ship’s company.

However, Ponting had drafted two papers stating that the cruiser had altered course and was moving away from the zone.

When his papers, which became known as the Crown Jewels, were withheld from the Foreign Affairs Select Committee, Ponting, in July 1984, sent the two documents in the post to the Labour MP for Linlithgow, Tam Dalyell. The documents revealed that the Belgrano had been sighted a day earlier than officially reported, and was steaming away from the Royal Navy task force, and outside the exclusion zone, when attacked and sunk.

Dalyell had resigned from the shadow Cabinet over the reconquest of the Falklands and was as voluble opponent of Thatcher’s. He had spoken in Parliament against the action and was therefore an obvious destination for the “jewels”. Dalyell realised they were political dynamite.

He passed them to the select committee, whose chairman contacted the defence secretary Michael Heseltine, who then ordered a leak inquiry.

The papers were soon traced to Ponting, who was interviewed by two MoD police officers, who told him they were not sure an offence had been committed. They suggested he quietly resign.

However, ministers demanded his prosecution. It came in the wake the MoD’s embarrassment over the case of Sarah Tisdall, a junior civil servant jailed the year before for sending photocopies of documents concerning the deployment of US cruise missiles in Britain to The Guardian. On August 17, 1985 – 35 years ago next week – Ponting was charged under Section 2 of the 1911 Official Secrets Act.

His defence, before a security-vetted jury, was that he had leaked, and to an MP, in the public interest. The judge, Mr Justice McCowan, was having none of it – he had considered halting the trial and giving the jury a direction to convict – and in his summing up he said “the public interest is what the government of the day says it is”.

Ponting was convinced he was going to be jailed, so much so that he brought his toothbrush and the sayings of Buddha to court for the verdict. But when it came, which no doubt enraged the judge, he was found innocent. He immediately resigned from the Civil Service and weeks later published his first book, The Right To Know, then joining the politics and international relations department at Swansea University.

The acquittal also enraged the Conservative-dominated Commons and, in 1989, a new Official Secrets Act came into force, which removed the defence of acting in the public interest, on which Ponting had relied. Ponting was no archetypal rebel. After getting a First in history he joined the Civil Service and when Thatcher came to power in 1979, he led a team identifying savings which the MoD could make, showing how the military could save millions on stock and food supply costs, and questioning whether the armed forces needed 3,000 full-time bandsmen. Thatcher praised him in Cabinet, laughed at his jokes and put him forward for an OBE.

As an academic he was a prolific, somewhat controversial, revisionist historian.

His 1993 biography of Winston Churchill depicted him as a racist who, among other measures, wanted to forcibly sterilise the “mentally degenerate” and send tens of thousands of Britons to labour camps.

One reviewer said of the book: “Ponting shattered the Churchill illusion for his readers, leaving them little to piece together, just marble shards on the floor of his looted temple.”

He retired in 2004 and after the 2016 Brexit vote moved with his wife to Kelso where, on July 28 this year, he died.