IT was an utterly heart wrenching moment when Prince William and Prince Harry were pictured walking behind Princess Diana's coffin, at her funeral in September 1997.

And although the boys appeared with their father, Prince Charles and their uncle, Earl Spencer, Prince Philip also walked behind the late princess' coffin - and now, the reason why has been revealed.

In royal author, Ingrid Seward's latest book - My Husband And I: The Inside Story of the Royal Marriage, Seward writes about the heartbreaking weeks that followed Princess Diana's death.

In one excerpt, she explains that Diana's brother, Earl Spencer, reportedly believed he should be the only one to walk behind his sister's coffin - something the royal family are said to have disagreed with.

"In a mark of respect to an ex-wife and a departed mother, Prince Charles wanted to walk behind the cortege with William and Harry beside him," Seward explains.

"Another of the by-now interminable rows ensued over the telephone, which Spencer ended by hanging up on the prince...It was made clear to Spencer that, regardless of whatever private feelings he might harbour, the prince and his sons would walk behind the gun carriage bearing the princess's coffin."

However, there was one difficulty - persuading Prince William to walk, something which William only agreed to do, so long as his grandfather - who he was very close to - walked too.

Seward explained: "At first William flatly refused. Charles pleaded with him and said that it would be utterly wrong of him not to accompany them.

"...Prince Philip weighed into the argument and eventually William agreed to take part - but only on the condition that his grandfather walked beside him."

According to Seward, Princess Diana's relationship with Prince Philip is said to have grown sour, but William had always been incredibly close to his grandpa.

"Diana had grown to dislike Prince Philip intensely - and he her - but Prince William was devoted to the old man," wrote Seward.

"...Now, William wanted his grandfather at his side in what was certain to prove the most harrowing public engagement the young man had had to endure."

In an interview in 2017, William later admitted that walking behind his mother's coffin was "one of the hardest things" he'd ever had to do.