The Queen Mother cancelled her royal engagements at the beginning of 1967
The Queen Mother had colon cancer at the age of 66 and surgery removed a tumour, her official biography reveals.
The book, published on Friday, says she underwent the 90-minute operation in December 1966 at the King Edward VII Hospital in central London.
Clarence House said at the time she had undergone abdominal surgery to relieve a partial obstruction.
The book also reveals her likes and dislikes, from food and television programmes, to certain heads of state.
Biographer William Shawcross interviewed Sir Richard Thompson, the Queen's former physician, for the official version of the Queen Mother's life.
It says the Queen Mother spent more than two weeks in hospital after bowel surgery and when discharged, cancelled all her official engagements for the first three months of 1967.
It also discredits the long-held belief she was fitted with a colostomy bag.
Some years after the operation, the Queen's physician at the time, Sir Richard Bayliss, wrote to one of the Queen Mother's ladies in waiting, saying that the lie of the "colostomy" operation should be countered.
Mr Shawcross said: "But the rumours that the operation had included a colostomy persisted.
"Many people who had to endure that operation themselves derived comfort from the belief that even someone with as active a life as Queen Elizabeth could manage so well after such a difficult procedure."
The Queen Mother successfully beat the disease, dying in her sleep in 2002 age 101.
Charles and Diana
The biography also reveals the Queen Mother's "regret" at the Prince of Wales's decision to speak publicly about his failed marriage.
Prince Charles admitted adultery on national television in 1994 as he spoke to his biographer Jonathan Dimbleby.
During a series of interviews the Queen Mother made in the 1990s and later used in her biography, she stated: "It's always a mistake to talk about your marriage."
A few years before the prince's confession, his estranged wife Diana, Princess of Wales, had collaborated privately with Andrew Morton on his book that exposed her life within the Royal Family.
The Queen Mother found this "deeply shocking" and felt that "the washing of dirty linen in public was utterly abhorrent", according to William Shawcross.
Mr Shawcross also appears to suggest the Queen Mother favoured Charles during the marriage break-up, stating: "She did not cast the princess aside at this time but she gave her grandson as much emotional support as she could."
The books reveals much about the Queen Mother's personality and her foibles.
Drawing on countless letters written by her, it reveals a love of popular television comedies like Dad's Army, Fawlty Towers and Keeping Up Appearances.
Mr Shawcross said she increasingly turned to the shows as she got older and games of cards or singsongs with friends became less frequent.
Readers will learn that Winston Churchill, Margaret Thatcher and the poet Ted Hughes were among her favourite people. She also had a soft spot for Americans, Canadians, Australians and miners, calling the latter "real people".
Those who did not find favour included the Liberal Democrats and US President Jimmy Carter, who made himself particularly unpopular when he kissed her on the lips.
Mr Shawcross said she wrote of the encounter: "I took a sharp step backwards - not quite far enough."
The Queen Mother reportedly enjoyed eating a lobster dish known as oeufs Drumkilbo, but disliked frozen food, oysters and smoked salmon. She also hated discussions about money.