Sir Robert Wade-Gery: Cabinet Secretary during the Falklands War who relayed Thatcher's order to sink the 'General Belgrano'

Wade-Gery was deputy to the Cabinet Secretary Sir Robert Armstrong, and the two men shared the fast-paced task of keeping the War Cabinet functioning

Sir Robert Wade-Gery was secretary to Margaret Thatcher's War Cabinet during the Falklands conflict of 1982, and relayed to Fleet Headquarters the order from Chequers to sink the Argentine cruiser General Belgrano.

"We were having drinks before lunch and I was tapped on the shoulder and taken aside," he recalled. "There was a signal in from the captain of the submarine which said in effect, 'I've got the Belgrano in my sights in my periscope. Do I sink her or don't I, because I've got to take a decision one way or the other pretty quick?'

"So I came back and said that this had happened, and the Prime Minister ... took us into the little white drawing room, and then she went round the whole room, not only ministers but all the officials, and said, 'What do you think we should do?' And everybody said, 'Sink it.' She hadn't at this point expressed a view, rather uncharacteristically, and she said, 'Well, I agree with that too, so you'd better go and tell them to sink it.' And I remember going out of the room while they went back to their Sunday drinks, and ringing up... and saying, 'Sink it.' And about an hour later, they did."

Wade-Gery, who had been seconded from the Foreign Office to the Cabinet Office, was deputy to the Cabinet Secretary Sir Robert Armstrong, and on the outbreak of war the two men shared the fast-paced task of keeping the War Cabinet functioning, often waking the Prime Minister in the middle of the night for a decision. Just over a month earlier Lord Carrington, then Foreign Secretary, had told Wade-Gery: "We've got a bit of bother with a scrap merchant in the South Atlantic, but I don't think it will come to anything." Wade-Gery had duly gone on holiday, only to return to find that the Falklands had been invaded. Thatcher is said to have addressed the diplomat with a phrase that followed him through his career: "What does Wade-Gery think?"

The seeking of Wade-Gery's opinion had been a memorable habit with his Classics master at Winchester College, the teacher finding himself in awe of the brilliant, confident boy whose father Theodore was Oxford University's Wykeham Professor of Ancient History.

Wade-Gery went to New College, Oxford and took a double First in Mods and Greats (Classics and Ancient History). His abilities marked him out after he joined the Foreign Office in the economic relations department in 1951, the year he became a Fellow of All Souls. After his first assignment abroad, to Germany, he became private secretary to the Foreign Office's Permanent Under-Secretary.

Wade-Gery's star continued to rise by way of the Southern department, where he helped to negotiate independence for Cyprus in 1960, and the 1969 Duncan Committee on overseas representation, of which it appears he drafted much of the eventual report, including the definition of Britain's future as "a major power of the second rank". He was then seconded to the Bank of England, and in 1970 recruited by Victor Rothschild to the Central Policy Review Staff, a Cabinet Office "think tank".

He took part in negotiations with the US Carter administration about the future of Britain's nuclear deterrent, devised contingency plans for handling the miners' strike that finally erupted in 1984, and was a secret emissary to Dublin to prepare for talks between the British and Irish governments. He took up a suggestion that to avoid discovery – or even assassination – he should walk down a street past a certain door in a garden wall, and disappear into it on its being opened at a pre-arranged time. "It worked perfectly," he said later.

In between these assignments he had further postings abroad, the next after Germany being as commercial secretary to Tel Aviv, during which time he returned to England in 1962 to marry Sarah Marris, known as Sally, who he had met at a dinner party, and drove back with his bride from Gloucestershire to Jerusalem.

A few years later Wade-Gery was sent as Head of Chancery to Saigon. Their nine months there (1967-68) encompassed the turning point of the Vietnam War, the Tet Offensive, in which the Viet Cong captured targets inside Saigon including the American Embassy just across the street from its British counterpart.

"[It was] the only time in my life where I've ever been seriously under shot and shell," he recalled. By this period he and his wife had with them a baby less than a year old, and a toddler. The children were evacuated as soon as the airport was regained and the Embassy aircraft recovered. The flights out took a month; his wife waited until the last to go. The Embassy was run down to a skeleton staff, and when Wade-Gery went home on leave he was told not to return. Saigon eventually fell in 1973.

Though describing himself as a "very bad" linguist, Wade-Gery learned Spanish for his assignment as Minister in Madrid during which, in 1975, Franco died and King Juan Carlos took over. Under two "very nice" British ambassadors who, he recalled, let him virtually run the Embassy, he found this the most enjoyable of all his foreign postings. He and his family would go skiing and in the summer enjoy climbing in the central sierras.

After Madrid, he felt that being Minister in Leonid Brezhnev's Moscow, (1977-79) as the Soviet Union approached its last decade of existence, was "like going into a dark tunnel", but he did meet the poet Yevgeny Yevtushenko. His secondment back in London as Deputy Secretary of the Cabinet, the Falklands war period, followed; immediately after the conflict he went to Delhi as British High Commissioner. On this, his last posting, he oversaw a state visit by the Queen in 1983. The next year brought the assassination by a Sikh bodyguard of the Indian Prime Minister Indira Gandhi.

Wade-Gery had got to know Gandhi's son Rajiv – who would take over – very well, once having brought him and his wife Sonia together with Norman Tebbit and his wife Margaret: "They were the only two politicians I knew who had started their life as airline pilots. I also thought that they were both going to end up as Prime Minister of their respective countries... they talked about airlines all through lunch."

After the Foreign Office Wade-Gery joined Barclays de Zoete Wedd. He also served as honorary treasurer of the International Institute for Strategic Studies, and renewed his association with All Souls.

ANNE KELENY

Robert Lucian Wade-Gery, diplomat: born Oxford 22 April 1929; CMG 1979, KCMG 1983, KCVO 1983; married 1962 Sarah Marris (one daughter, one son); died Cold Aston, Gloucestershire 16 February 2015.