The letter, which has been hidden in the Vatican for nearly 500 years, is to go on display at the British Library in London in April.

Henry wrote to Anne Boleyn, who would become his second wife, in January 1528, apologising to her for suggesting that she become his mistress.

In the letter Henry promises Anne that "henceforth my heart will be dedicated to you alone", an assurance that contradicts Henry’s womanising reputation.

Henry continues, "The beautiful words of your letter are so cordially phrased, that they really oblige me to honour, love, and serve you for ever."

Henry went on to marry Anne Boleyn after divorcing his first wife, the Spanish princess Catherine of Aragon, but despite having written to Anne like a lovesick schoolboy, Henry’s marriage to her ended badly.

Having failed to provide him with a male heir, Anne fell from favour with the king, whose chief minister had her charged with treason and adultery.

Anne was executed by beheading at the Tower of London in May 1536, leaving the way clear for Henry to marry his third wife Jane Seymour.

According to contemporary witnesses Anne went to her death displaying queenly dignity; on the morning of her execution she told her prison guard, "I heard say the executioner was very good, and I have a little neck."

Henry went on to be married a total of six times, divorcing twice and executing another wife, but in spite of all his efforts, Henry died without leaving a surviving male heir, a situation which resulted in the coronation of Anne’s daughter, Queen Elizabeth I.