“It’s still raw,” he told a documentary filmmaker last year on the 20th anniversary of Diana’s death. He added: “It’s incredibly hard. I’ll have to sort of deal with that for the rest of my life.”

Tonight he deals with it in his marriage to Meghan Markle by honouring his mother in countless ways through the day.

He has invited her family, the Spencers — including provocative Uncle Charles, 9th Earl Spencer, who once declared himself the “protector of the princes” — to attend the wedding.

He has asked Diana’s sister Lady Jane Fellowes to present a reading. Diana was just 36 — three years older than Harry is now — when she died of injuries sustained in a shocking car accident on a Paris motorway in 1997.

Harry was 12 at the time, his brother William, 15 — both only boys unable to process the immense international outpouring of grief that followed.

Had she survived the accident, Diana, proud mother of the prince who today is considered the royal family’s most popular member, would have been a youthful 56 — still young and captivating enough for cameras to be trained on her on this rarest of days, a public coming together of Windsor and Spencer clans.

Her every knowing smile and cheeky jape at St George’s Chapel, the venue that hosted Harry’s christening in 1984, would have been recorded for public consumption.

In life, the fashion-loving princess held the title of most photographed woman on the planet. The public’s fascination with her style and beauty was boundless.

In death, the legend of Diana, the “People’s Princess”, royal outcast, has grown, creating a pall the royal family has learned to live with.

Intrigue surrounding her untimely and ghastly death may have fanned the legend but to her sons she remains something more illusive, their irreplaceable “mummy”.

Diana has been absent from the most important milestones in her sons’ lives. She missed their first clumsy foray into dating, their falling in love, their growing into manhood, their struggle to find relevance in a family that many believe no longer has any, their weddings and eldest son William becoming a father.

Speaking of his own enduring grief, William last year noted that had Diana lived to be grandmother to his children with Kate Middleton “ … she’d love the children to bits”.

Sadly William and Harry’s children will never know the love of the adventurous, playful grandmother Diana might have been, while their wives, Kate and Meghan, will miss the benefits of having her as a mother-in-law, a role she might have enjoyed.

Though she never mastered them herself, Diana knew something of the survival skills needed to find one’s place in what Prince Philip once called “The Firm”, the foreboding royal family. She could have proven a useful coach and ally to Kate and Meghan. But perhaps her greatest legacy to her sons will be the example of her own failed marriage.

Having seen first hand the disastrous match Charles and Diana made for themselves — the marriage ended after 11 years in 1992 when the boys were seven and 10 — Harry and William have picked wives who would have once tested orthodox royal marital convention.

Diana was only 20 when she married 32-year-old Charles and, despite her aristocratic background, the couple hardly knew one another. She soon toppled into depression in a loveless union.

In contrast William spent a decade getting to know his future wife Kate, eight of those in courtship. He was 28 when he married, his bride 29.

Harry, too, has taken his time choosing a bride. Though they’ve only known each other two years, Harry has looked for maturity and worldliness in his match, qualities Diana did not possess when she married Charles.

Like Kate, Meghan, at 36, is also older than Harry, 33 — an example set by Prince Charles, 69, and his second wife, Camilla Parker Bowles, 70. American divorcee Meghan bucks all royal convention concerning princess brides — older, American, divorced — proving that even the Queen has learned her lessons from Diana, whose ghost still hangs large over the royal family and its descendants.