In the memorable one-to-one with Martin Bashir, Diana uttered the famous line “there were three of us in this marriage” when referring to Charles and his long-term love Camilla, now the Duchess of Cornwall.

The interview, which was watched by 22.8million viewers, caused huge embarrassment for the royal family after she also said the monarchy was in desperate need of modernisation and called the prince’s camp the “enemy”.

It plunged the royals into crisis as the general public sided with Diana – prompting the Queen to write the letter, which left the princess in no doubt about her annoyance at the interview.

The Queen wrote in the letter: “I have consulted with the Archbishop of Canterbury and with the prime minister and, of course, with Charles, and we have decided that the best course for you is divorce.”

It reportedly left Diana furious as she felt she was being forced to agree to a divorce – despite not wanting one.

Days later, Buckingham Palace formally announced the divorce in a statement saying “after considering the present situation the Queen… gave them their view, supported by the Duke of Edinburgh, that an early divorce is desirable”.

The following year on August 28, 1996, the divorce was complete. It brought to an end the marriage of the pair that began with a huge fairytale ceremony at St Paul’s Cathedral on July 29, 1981.

As part of a settlement, Charles and Diana agreed to share custody of their sons, Prince William and Prince Harry.

Diana was reportedly awarded a lump sum of £17million along with £350,000 each year to run her private office. She was also allowed to keep her apartments at Kensington Palace.

However, one thing she was stripped of was her title of Her Royal Highness. She then became known as Diana, Princess of Wales.

This led to a heartbreaking conversation between Diana and a then 14-year-old William that left her in tears, according to her butler Paul Burrell.

The young prince reportedly told his mum: “Don’t worry Mummy, I will give it back to you one day when I am King.”

However, William’s promise to give his mother back her HRH title was made even more heartbreaking almost exactly a year after the divorce was finalised.

Diana died following a car crash in Paris on August 31, 1997, alongside her companion Dodi Fayed – with this year the 25th anniversary of her shocking death.