The Queen broke this established royal custom in tribute to Princess Diana

Today marks the 24th anniversary of Princess Diana's tragic death in 1997. Beloved by the public for her iconic style and humane approach to royal duty, the world was plunged into grief following news of the Princess's fatal car accident in Paris on this day 24 years ago.

Though her sons, Princes William and Harry, are thought to be marking the anniversary privately, the public have been granted special access to the Princess Diana memorial at Kensington Palace's Sunken Garden to pay their respects.

William and Harry, then aged 15 and 12, were on holiday at Balmoral with the Queen at the time of their mother's death. It was agreed that the Royal Family should stay at Balmoral to grieve the princess in private – but the royals' delay in addressing the Princess's death began to anger the public, and eventually lead to the Queen breaking royal protocol in one very significant way.

Prior to Diana's death, it was established royal custom that the Royal Standard flag would be flown over Buckingham Palace only if the monarch was present. As a result of the Queen's absence at her London residence following Diana's death, no flag was on display over the palace.

However, in the days after Diana's death, there was a growing feeling that a flag should be flown over Buckingham Palace at half-mast as a mark of respect. Her Majesty eventually decided to break with royal protocol entirely, by agreeing that the Union Flag could be flown at half-mast over Buckingham Palace on the day of Diana's funeral.

This significant change wasn't just a temporary measure, though. Since Diana's death, the Union flag flies at Buckingham Palace when the Queen is not in residence. When she is there, it is replaced with the Royal Standard.

The Union Flag has been lowered to half mast as a mark of respect on several occasions since – including the death of the Queen Mother in 2002, the September 11 attacks in 2001, and the July 7 London bombings in 2005.

The flag was also flown at half-mast on the first anniversary of Diana's death on this day in 1998.

Though William and Harry reunited earlier this summer to unveil the much-publicised statue paying tribute to their mother at Kensington Palace, it's thought that the brothers will this year miss out on their long-held tradition of marking the anniversary together.