On June 3, 1991, Princess Diana was having lunch with a friend at her preferred Italian spot in Knightsbridge, when their meal was unexpectedly interrupted by her bodyguard.

Her son, Prince William, who was eight at the time, had sustained a ‘serious injury’ to his head while playing with a golf club alongside a classmate at their private boarding school, Ludgrove, located in Berkshire.

With an apology to her friend, Diana quickly left the restaurant, while Prince Charles set out on the journey from Highgrove to the Royal Berkshire Hospital in Reading, where their eldest son was undergoing tests.

When both parents arrived, the doctors suggested William be taken to the Great Ormond Street Hospital for Sick Children in London.

Thankfully, the young royal was ‘chirpy and chatty’ as he rode in the ambulance alongside his mother, with his father following closely behind in his Aston Martin sports car.

Doctors, including the Queen Elizabeth’s physician Dr Anthony Dawson and neurosurgeon Richard Hayward, soon found that William had suffered a depressed fracture of the skull and required an immediate operation.

‘They made it clear that there were potentially serious risks, albeit relatively small, both in the operation and in the possibility that the Prince could have suffered damage to the brain during the initial accident,’ royal biographer Andrew Morton wrote in Diana: Her True Story – In Her Own Words.

But instead of staying with his wife, and, more importantly, standing by his eldest son’s side, Charles left the hospital to attend a performance of Puccini’s Tosca at the Royal Opera House in London’s Covent Garden.

His decision to put duty before family may have come as a shock to the public – but it did not surprise his wife. She accepted his decision to go to the opera as ‘nothing out of the ordinary’, according to Morton.

William, holding his mother’s hand, was wheeled into surgery for the 75-minute operation.

Diana then waited anxiously in another room until Dr Hayward emerged to tell her William was fine. She later said it was one of the longest hours of her life.

As she sat with William in a private room watching nurses come and go every 20 minutes to test his blood pressure and shine a light in his eyes, Charles boarded the royal train for an overnight journey to North Yorkshire where he was due to attend an environmental study.

Although a Buckingham Palace spokesperson said he stayed in close touch with doctors, the media quickly cottoned on to the fact that Charles was not with his recovering son.

Pictures of the prince wandering the Yorkshire Dales on his green mission were plastered across front pages. ‘What kind of dad are you?’ asked the Sun’s headline.

Diana, on the other hand, saw this as yet another example in a continuing pattern.

Morton wrote that a close friend who spoke to her as soon as William came out of the operation room said: ‘Had this been an isolated incident it would have been unbelievable. She wasn’t surprised.

‘It merely confirmed everything she thought about him and reinforced the feeling that he found it difficult to relate to the children. She got no support at all, no cuddles, no affection, nothing.’

James Gilbey, Diana’s lover and the man behind ‘Squidygate’, reinforced this view: ‘Her reaction to William’s accident was horror and disbelief. By all accounts it was a narrow escape.

‘She can’t understand her husband’s behaviour so, as a result, she just blocks it out. Diana thinks: “I know where my loyalties lie: with my son.”‘

Following two anxious nights spent at the hospital, William was discharged and travelled home with his mother.

The bandages had been removed, revealing a noticeable bump and a line of stitches on the young prince’s head.

When Charles became aware of the public’s disdain, he allegedly blamed Diana for making an ‘awful nonsense’ about the severity of William’s injury.

He also claimed he was unaware that his son and heir to the throne could have suffered brain damage.

‘The dramatically different manner in which the couple responded to William’s injury publicly underlined what those within their immediate circle had known for some time: the fairytale marriage between the Prince of Wales and Lady Diana Spencer was over in all but name,’ wrote Morton.

Charles told his official biographer Jonathan Dimbleby that he picked things back up with Camilla in 1986.

Meanwhile it is alleged Diana’s affair with army captain James Hewitt started around the same time.

The Prince and Princess of Wales, who had separate bedrooms at their homes for years stopped sharing the same sleeping quarters during an official visit to Portugal in 1987.

The next few years saw Charles and Diana plagued with rumours of marital problems, culminating in the blistering tell-all of the collapse of their marriage as published by Morton in 1992.

The doomed royal couple announced their separation just months after the publication of Diana: Her True Story – In Her Own Words and finalised their divorce in August 1996. 

As well as the scar of his parent’s divorce, William was left with a visible scar running across his forehead, which he humorously refers to as his ‘Harry Potter’ scar.

He told the story of the injury while he was being interviewed by a 10-year-old cancer patient for CBBC’s Newsround in 2009.

Speaking to Alice, a patient at the Royal Marsden Hospital, he pointed at his head, saying: ‘That was for my Harry Potter scar, as I call it, just here.

‘I call it that because it glows sometimes and some people notice it – other times they don’t notice it at all.

‘I got hit by a golf club when I was playing golf with a friend,’ he explained.

‘Yeah, we were on a putting green and the next thing you know there was a seven-iron and it came out of nowhere and hit me in the head.

‘I was in hospital, but that was very minor compared to how many times you’ve been into hospital.’