​ROBERT JOBSON: William and Harry may understand their mother better now. But not enough to honour their promise to her...

Diana, Princess of Wales, bestowed upon her boys a singular charge: 'You must promise me that you will always be each other's best friends,' she told them.

Back then the brotherly pledge seemed easy.

Their accord was sealed with high-fives and a maternal hug. It speaks volumes of the late princess's aspirations for William and Harry.

Sadly, that vow, revealed in royal historian Robert Lacey's bestseller, Battle of the Brothers, has failed to withstand the tumultuous tests of time.

More than a quarter of a century after Diana died in a car crash in Paris, her 'boys' are now men and barely on speaking terms.

Princess Diana, with Prince William and Prince Harry on holiday in Majorca in 1987

After what has gone on in recent years, there seems little prospect of the bitter feud between them healing any time soon.

A new statue at Kensington Palace, a silent testament to their mother's legacy, drew them together briefly in July 2021, a pause in their estranged relations, on the eve of what would have been Diana's 60th birthday.

But it was a fleeting truce in a narrative marked by distance and discord.

Now tonight at The Diana Award – a worthy charity that mirrors the essence of their mother's vision and a belief in the transformative power of youth – the brothers will make a rare joint appearance to honour her at the Science Museum in London.

Neither, however, will be in the room at the same time – clearly a stipulation – not even virtually.

William will give a speech in person to mark the charity's 25th anniversary and present awards to 20 recipients.

Harry will join a video call with the winners, according to reports, but only after his big brother has left.

In the past they often appeared together in person to pose with the award winners.

Since Harry's 2018 wedding to his wife Meghan, relations between the brothers have soured

The brothers at their mother Diana's funeral. Her parenting was grounded in love and hugs

What would she have made of it all?

'She would have banged heads together, she wouldn't have put up with it,' said one ex loyal retainer.

Diana's parenting, grounded in love and hugs, was also one of discipline. She would hand out the odd smack or clipped ear if she felt the situation warranted it. She gave nannies permission to do the same.

It shaped William and Harry with a blend of royal duty and human frailty.

There was lots of laughter, and a few tears, the insider said. She made the boys do mundane chores, showing how she wanted then to grow up with a sense of normalcy, despite their royal rank.

She clashed with staff who reminded them that they were special.

Growing up William and Harry were inseparable. They relied on each other in the aftermath of their parents' estrangement and divorce.

Despite the normal sibling rivalry, they always had each other's backs, through their school years, where Harry followed William to Ludgrove and then Eton.

Harry looked up to his brother, William always looked out for Harry.

Their brotherly relationship set against the backdrop of loss, love, and legacy, was always strong.

Growing up William and Harry were inseparable. They relied on each other in the aftermath of their parents' estrangement and divorce

Who can forget the images of them at her funeral as they struggled to cope with the colossal loss of their iconic mother.

Hearts across the world went out to the grieving sons she helped shape.

Robert Lacey also revealed in his book that Diana's friend, Simone Simmons, recalled that the princess also told her boys: 'Never let anyone come between you.'

'Both boys then promised they would keep that vow and then they went out to play football,' Lacey reports.

It was typical Diana. Direct. She had a way with them, straightforward and firm when needed.

Critics of her approach missed the point. They said she smothered them with love. But Diana was moulding princes.

William and Harry, parents themselves now, may be understand their mother better. But not enough…

Reconciliation seems a distant hope.

The chasm between them reflects the complexity of their lives and the emotional burden they carry.

It is sad but perhaps not surprising that their paths diverged. The weight of history and personal grief under the scrutiny of millions has marked the journey of two princes who seem unable to honour to the promise they made their mother long ago.