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It’s a day in history that will never be forgotten.

At just 15 and 12 years old, Prince William and Prince Harry walked behind their mother Princess Diana’s coffin at her funeral in 1997.

But the young princes didn’t walk alone.

Not only did their father, Prince Charles, and their uncle, Diana’s brother Earl Spencer join them – their grandfather Prince Philip walked too.

For years, many questioned why Prince Philip would choose to walk behind the coffin, as he was said to have had an “intense dislike” for Diana.

But now, in royal author Ingrid Seward’s latest book My Husband and I: The Inside Story Of The Royal Marriage, the reason behind Prince Philip’s baffling decision has finally been explained.

Originally, Diana’s brother Earl Spencer wanted to be the only person to walk behind Diana’s coffin.

"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 wrote.

"Another of the by-now interminable rows ensued over the telephone, which Spencer ended by hanging up on the prince," she continued.

"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."

But while Earl Spencer and Prince Charles eventually settled their disagreement, Prince William, who was 15 at the time, refused to walk behind his mother's coffin without his grandfather Prince Philip beside him.

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

Although Prince Philip and Diana's relationship had fallen apart, Prince William and his grandfather were said to be incredibly close.

Eventually, Prince William agreed to take part in walking behind his mother's coffin on the condition that Prince Philip would walk beside him.

And despite his detest for Princess Diana, Prince Philip agreed.

According to a BBC documentary about Diana's death released last year, discussions about whether William and Harry should walk behind the coffin continued until just hours before the funeral took place.

In the documentary, Diana's two sons also spoke about what it was like when their father told them that their mother had passed away.

"One of the hardest things for a parent to have to do is to tell your children that your other parent has died," Prince Harry said.

"But he [Charles] was there for us, he was the one out of the two left and he tried to do his best and to make sure we were protected and looked after."

Prince William added that he recalled feeling "disorientated, dizzy and very confused" upon hearing the news.

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