A week before her official biography goes on sale, Buckingham Palace has allowed the publication of an emotional account by the Queen Mother of the day 69 years ago when a Nazi attack could have killed her

It would go down in history as the day the Luftwaffe came closest to claiming the ultimate trophy – the life of George VI. Exactly 69 years ago today, German bombs hit Buckingham Palace when he was in residence, an event elevating the reluctant, stammering monarch to hero king in the eyes of the people.

Now a private and emotional letter written by Queen Elizabeth the Queen Mother, only hours after they both survived the attack, can be reproduced in full for the first time.

Released by Buckingham Palace ahead of the publication this week of the first official biography of the Queen Mother, the letter is her personal account of the events of 13 September 1940 to her "darling" mother-in-law, Queen Mary.

In it she records how she was "battling" to remove an errant eyelash from the King's eye, when they heard the "unmistakable whirr-whirr of a German plane" and then the "scream of a bomb".

"It all happened so quickly that we had only time to look foolishly at each other when the scream hurtled past us and exploded with a tremendous crash in the quadrangle," she wrote.

While her "knees trembled a little bit", she was "so pleased with the behaviour of our servants", some of whom were injured as one bomb crashed through a glass roof and another pulverised the palace chapel.

Hours later, after lunching in their air-raid shelter, she and the King were visiting West Ham in London's East End. She wrote: "I felt as if I was walking in a dead city... all the houses evacuated, and yet through the broken windows one saw all the poor little possessions, photographs, beds, just as they were left."

The bombing, along with the royal family's refusal to flee Britain against Foreign Office advice, was to win the King and Queen affection and fellow-feeling across the country. The Queen declared: "The children will not leave unless I do. I shall not leave unless their father does, and the king will not leave the country in any circumstances, whatever."

The letter, written on Windsor Castle headed notepaper, is one of thousands to which official biographer William Shawcross was given unrivalled access for his long-awaited biography, published seven years after the Queen Mother's death in 2002, aged 101.

Famously discreet in public – she gave only one interview in her life, on her engagement to her "beloved Bertie" – the book also draws on taped conversations she had with her confidante and former Eton College headmaster, Sir Eric Anderson. These were interviews conducted at the instigation of Prince Charles, so she could commit her memories to print for posterity.

Shawcross, a close friend of Charles who received a reported £1m advance, was appointed by the Queen, who read and approved the manuscript.

The biography is expected to give further insight into the strained relationship between the Queen Mother and Wallis Simpson, the American divorcee whose relationship with, and subsequent marriage to, Edward VIII rocked the establishment, leading Edward to abdicate and "Bertie" to take the throne. With no love lost between the two – Simpson described Queen Elizabeth as the "fat Scotch cook" responsible for their exile, while the Queen refused to receive her and declared that the country "hated" her – anticipation is high.

It will also examine how Lady Elizabeth Bowes-Lyon, the country aristocrat, steeled herself for queenhood only to lose her status with the premature death of Bertie, and reinvented herself as the country's favourite "grandmother" for the remaining half-century of her life. Observers hope it will also shine a light on her political influence over the King.

Waspish, with a fondness for horses, fishing, and (allegedly) Dubonnet and gin, the Queen Mother's own voice has been little heard, despite her longevity. Snatches so far have given interesting insights. The writer AN Wilson's indiscreet disclosure of her dinner-party chat revealed a woman who complained about her overdraft, was bitchy about Prince Michael of Kent, and admired Conservative governments.

In another letter, written during the war to Prince Paul Karageorgevic, Prince Regent of Yugoslavia and a member of the huge extended royal clan, she makes lighthearted fun of her flamboyant royal photographer, Cecil Beaton. "I am ordering a photograph for you," she writes. "And Mr Beaton, who is mincing away at some light war work, will execute my orders as soon as possible.

"I believe he is a telephone operator," she adds, in a letter now in Prince Paul's archives. "Can you not imagine him saying 'Number, darling! 2305. Oh, divine, my dear, etc, etc'."

More than anything, the war defined the Queen Mother, and the book promises to plough this rich furrow thoroughly. The bombing of the palace led the Queen Mother to utter one of her most famous comments: "I am glad we have been bombed. Now we can look the East End in the eye."

In her letter, written after a tiring day of drama, she confides: "It does affect me, seeing this terrible and senseless destruction – I think that really I mind it much more than being bombed myself. The people are marvellous, and full of fight. One could not imagine that life could become so terrible.

"We must win in the end."