As the gun carriage bearing Princess Diana's coffin passed by, a figure dressed in black bowed her head.

It marked the first and only time in her life that Queen Elizabeth would ever bow to another person in public - breaking all the rules.

It wasn't just that the monarch was acknowledging Diana, it was the Queen acknowledging her people, reports The Sun.

The BBC captured the only moment the Queen bowed her head to anyone. Photo / BBC

Following Diana's death on August 31 in 1997, the monarchy was changed forever. The Queen had never faced such outrage from a country stricken by grief, incredulous at her apparent indifference, symbolised in particular by the flag at Buckingham Palace not flying at half-mast.

Suddenly, the Queen's priority was to be a grandmother to William and Harry instead of putting her duties first. But once the shock passed, she faced the public again with a moving TV broadcast.

She then came out of Buckingham Palace to join the crowds gathered to watch Diana's coffin as it made its way to Westminster Abbey - and she bowed her head.

The monarch had been at Balmoral with her family, along with William and Harry, when the call came from Paris at 4.15am: Diana was dead.

As a grandmother, the Queen's first thought was for Diana's sons. William was 15 and Harry was just 12, and both were asleep. She decided not to wake them immediately, and ordered staff not to turn on TVs or radios in case they heard anything, and to hide any newspapers.

Just after 7am, Prince Charles woke them up and broke the news. An hour later, the distraught father met with his mother the Queen to decide on their next steps.

Despite having been divorced the previous year after separating in 1992, Charles was determined to go to Paris to collect Diana's body.

But the Queen believed it was more important for him to remain in Scotland with his sons.

It was at this point that one of the royal household staff interrupted, saying, "Would you prefer, Ma'am, that the body of the Princess of Wales be brought home in a Harrods van?"

While it was the first angry voice raised against the monarch for her seemingly cold reaction to Diana's death, it would not be the last.

Eventually Charles got his way, but not before the Queen decided that the family would attend church as usual.

Diana was not mentioned during the service.