It was the speech that came to define the late Queen’s 70 years on the throne.

Speaking to the nation from Cape Town on her 21st birthday in 1947, Princess Elizabeth promised Britons that her ‘whole life whether it be long or short’ would be devoted to their service.

Royal writer Valentine Low revealed in his 2022 book Courtiers: The Hidden Power Behind the Throne that the future Queen was made so emotional by the stirring words that she cried.

Alan Lascelles, private secretary to her father King George VI, said to her in response: ‘Good, for if it makes you cry now, it will make 200million other people cry when you deliver it, and that is what we want.’

Incredibly, a draft of the speech was briefly lost in a bar in Cape Town the month before Elizabeth uttered the words.

When it was found, Lascelles wrote to Dermot Morrah, the journalist who wrote the speech, to tell him: ‘The missing letter has now turned up. The steward in the Protea diner had put it in the bar, among his bottles, little knowing that it was itself of premier cru.’

He went on to praise Morrah for the quality of the speech. Lascelles said: ‘I have been reading drafts for many years now, but I cannot recall one that has so completely satisfied me and left me feeling that no single word should be altered.

‘Moreover, dusty cynic though I am, it moved me greatly. It has the trumpet ring of the other Elizabeth’s Tilbury speech, combined with the immortal simplicity of Victoria’s “I will be good”.

Elizabeth spent six months in South Africa with her father, mother Queen Elizabeth and sister Princess Margaret.

The tour came just two years after the end of the Second World War, at a time when the British Empire was being dismantled.

Low reveals how, in describing the success of the tour, Lascelles wrote in his diary: ‘The most satisfactory feature of the whole visit is the remarkable development of Princess Elizabeth.

‘She has come on in the most surprising way, and all in the right direction.’

He added that she had a ‘good, healthy sense of fun’ but could also ‘tale on the old bores with much of her mother’s skill.’

Morrah, who had previously penned speeches for George VI during the Second World War, also published a book about Elizabeth to mark her birthday.

Elizabeth began her birthday speech, which was delivered from Government House in Cape Town, by saying: ‘On my twenty-first birthday I welcome the opportunity to speak to all the peoples of the British Commonwealth and Empire, wherever they live, whatever race they come from, and whatever language they speak.

‘Let me begin by saying “thank you” to all the thousands of kind people who have sent me messages of good will. This is a happy day for me; but it is also one that brings serious thoughts, thoughts of life looming ahead with all its challenges and with all its opportunity.

At such a time it is a great help to know that there are multitudes of friends all round the world who are thinking of me and who wish me well. I am grateful and I am deeply moved.’

She went on to mention the five-year conflict with Nazi Germany, saying: ‘We must not be daunted by the anxieties and hardships that the war has left behind for every nation of our commonwealth.

‘We know that these things are the price we cheerfully undertook to pay for the high honour of standing alone, seven years ago, in defence of the liberty of the world.

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