It has gone down in history as the key speech of the Queen’s long life of service, laying out her pledge to devote herself to Commonwealth duties forevermore.

Delivered on her 21st birthday, as Princess Elizabeth undertook a major tour with her parents in South Africa, it is said to have reduced Sir Winston Churchill to tears as he listened, becoming known in shorthand as the “Cape Town speech” ever since.

Newly-public evidence, uncovered in the diaries, letters and tour schedules of the Royal Archives, has now identified a “fascinating little mystery” over the speech, indicating it was not delivered live or indeed recorded in Cape Town at all.

In fact, research suggests, it was carefully pre-recorded a week earlier at a hotel in what is now Zimbabwe, allowing the young Princess to read it without fear of ruining her birthday under pressure.

It is now regularly quoted in books, television programmes and newspaper articles about the Queen for its most famous line: “My whole life whether it be long or short shall be devoted to your service and the service of our great imperial family.”