Mollie Butler gradually became one of the dwindling band whose voices whispered the last enchantments of Edwardian England. To listen to her speak even on television — and she was still doing so at the age of 90 — was to hear English as it was once spoken in the imperturbable confidence of an imperial age.

Her intonations and inflections — as heard in her invariable reference to young women as “gels” — were not so much cut glass as polished mahogany; and no one ever brought a more gracious air to the now neglected art of social entertaining.

It was in 1959 that Mollie Courtauld, as she then was (through her earlier marriage to August Courtauld), became the second wife of Rab Butler, then Home Secretary, Leader of the House of Commons and chairman of the Conservative Party. It was to prove an idyllic, happy match — and for the rest of her life Mollie was to be the zealous keeper of the Rab flame.

She never forgave Harold Macmillan for, as she believed, deliberately and maliciously blocking her husband’s claim to the prime ministership in the Tory leadership crisis of 1963 — and went out of her way, even in old age, to insist that she had found Macmillan totally untrustworthy. It was, she said, her view from the moment that she had first met him.

Her antipathy towards Macmillan was not just a matter of words; it was reflected in her actions. When Rab invited his old chief and rival to stay in Cambridge with him at Trinity (where, greatly helped by Mollie, he served as a highly successful Master from 1965 to 1978) his wife not only refused to have Macmillan in the Master’s Lodge — he had to be banished to the adjoining Judges’ Lodgings — but steadfastly refused to speak to him, even when summoned to chauffeur him the short distance to the Cambridge Union, where the former Prime Minister was due to attend a debate.

To a subsequent inquiry as to whether saying nothing had not proved rather awkward, she — totally unchastened — replied: “No, I simply thought of those splendid gels who used to drive generals about during the war and kept my eyes firmly to the front.”

Mollie Montgomerie, as she was born, was the elder daughter of Frank Douglas Montgomerie, of Castle Hedingham, Essex. She was brought up very much as a child of her time — being taught mainly by governesses, though going briefly to school during the First World War at a day academy run by two elderly spinster ladies on Wimbledon Common. (She and her sister were at the time staying with their maternal grandmother, who lived just off the Common.)

She also spent some time learning French at a finishing school in Lausanne. Back at home in Essex she became a friend of Betty Courtauld of the textile family, to whose brother August — after the normal cursus honorum of May Balls, point-to-points and the like — she became engaged in 1930, marrying him (after his great adventure beneath the ice cap in Greenland) in 1932.

As well as being an Arctic explorer, August Courtauld had a passion for boats, causing his wife to write wryly of him in her wonderfully evocative memoir August and Rab (1987) that “life with August was to consist largely of life at sea”.

With their base at Spencers, Great Yeldham, Essex, they found time, however, to bring up six children, and it was these who first brought her into contact with Rab who — married to the Courtauld heiress, Sydney — lived not far away at Stanstead Hall.

In later life Mollie would often confess that she found ringing up to make school holiday arrangements with her children’s second cousins at Stanstead Hall something of an ordeal — as she was always terrified that Rab would answer the phone and she would not know what on earth to say to so lustrous a political luminary. The same went for her dread of being placed next to him at meals.

She was always quite specific as to when and how her attitude changed. Her first husband had been struck down with multiple sclerosis in 1953, and Rab’s wife Sydney was at the same time dying a painful death from cancer of the jaw.

On November 30, 1954, having spent the morning at Eton for St Andrew’s Day, she went to Westminster Hall to watch the 80th birthday presentation of the ill-fated Graham Sutherland portrait to Winston Churchill. Spotting a grief-ridden Rab sitting forlornly on the dais — he was at the time Chancellor of the Exchequer — she suddenly, in her own words, “realised that I loved that man”.

There were, however, practical difficulties in the way of any too close public association. Although Sydney died less than a fortnight later, August Courtauld — with his brain increasingly affected by the development of his illness — survived until March 1959. Six months later Rab and Mollie were married — with the latter replying, with some asperity, to those who thought the timing a little hurried that “they did not know how long ago August had left me”.

The marriage, which lasted until Rab’s death in 1982, provided an emotional Indian summer for both of them — though there were those who suspected that, had Sydney lived, Rab would infallibly have become Prime Minister. Certainly, where Sydney supplied what Rab called in his autobiography “astringent spurs”, Mollie tended to wrap her husband in a warm and comfortable eiderdown.

Personally to the left of Rab on most social and political issues — including capital punishment, homosexuality and sporting sanctions against South Africa — she was never prepared to consider or countenance any criticism of him.

In 1995 — eight years after her August and Rab — she privately published a book that she entitled A Rabanthology. She also worked on a volume of the letters that had passed between them. She finally, however, concluded that this correspondence was too private to be published and, instead, made arrangements for the letters to be deposited alongside Rab’s own papers at Trinity College, Cambridge.

She is survived by her own four sons and two daughters, together with two stepsons and a stepdaughter. A third stepson, Sir Adam Butler, who was the Conservative MP for Bosworth from 1970 to 1987 and also served in several departments as a Government minister, predeceased her.

Lady Butler of Saffron Walden, widow of R. A. Butler and author of August and Rab, was born on September 10, 1907. She died on February 18, 2009, aged 101