As a member of the Royal Enclosure, I can attest that there was only one subject on the lips of Royal Ascot racegoers yesterday – and it wasn’t any of the horses.

The talk among the elegantly dressed ladies and top hat-wearing gentlemen in the Berkshire racecourse’s most exclusive area was about the absence of the Princess of Wales.

Yesterday her husband Prince William presented the trophy to the winner of the Prince of Wales stakes, which was due to be a big public occasion for the couple.

So much so that they had invited some of their closest friends to join them in their box including property mogul Thomas van Straubenzee – godfather to their daughter Princess Charlotte – and his wife, Lucy – a teacher at Thomas’s school in London, which counts Charlotte and her brother Prince George as former pupils.

Other friends in their box included James Meade, who has been one of William’s closest friends since their schooldays at Eton College, and Sam Waley-Cohen, a key figure in the couple’s love story.

Sam, a healthcare boss and Grand National-winning amateur jockey, is credited with reuniting the then-Kate Middleton and William at his Oxfordshire home after they split up in 2007.

The couple’s other friends had good reason to look forward to Catherine joining them yesterday as her presence in the Royal Procession had been formally announced by the racecourse at midday.

Catherine was listed as riding in the second carriage from Windsor Castle with William, golfer Justin Rose and his wife Kate.

Catherine appeared at the Order of the Garter celebrations in Windsor on Monday

Yesterday Prince William presented the trophy to the winner of the Prince of Wales stakes, which was due to be a big public occasion for the couple

However, just 22 minutes after Royal Ascot posted the carriage list online, it published a second list with different details – and no Catherine.

William had been switched to the first carriage with King Charles, Queen Camilla and Prince Saud bin Khalid al-Saud, a key member of Saudi Arabia’s ruling family.

It was the second consecutive day at Ascot for the King, who is still undergoing cancer treatment.

But the sudden change caused widespread confusion and some alarm given that Catherine, who appeared at Trooping the Colour on Saturday and at the Order of the Garter celebrations in Windsor on Monday, only announced she was in remission from an undisclosed form of cancer in January.

William seemed in good spirits, though, and the attendance of Catherine’s mother, Carole Middleton, as well as her sister-in-law, Alizee Thevenet, suggested there should not have been great cause for concern.

Kensington Palace officials declined to comment. However sources said that Catherine was ‘disappointed’ not to attend but had ‘to find the right balance as she fully returns to public-facing engagements’.

The sources blamed the racecourse for the confusion, insisting that the Palace had notified Ascot earlier that Catherine would not be attending. ‘Unfortunately, an inaccurate version of the carriage list was issued in error after that notification,’ an insider added. ‘The correct list was then circulated.’

The confusion had unfortunate echoes of an incident in March last year when the Ministry of Defence caused some embarrassment by announcing that Catherine would attend a parade rehearsal for Trooping the Colour in her role as Colonel of the Irish Guards.

Sam and Annabel Waley-Cohen and Thomas and Lucy van Straubenzee were all invited into the royal box

Also in attendance were the Princess of Wales's mother, Carole Middleton, and her sister-in-law, Alizee Thevenet

Kensington Palace did not confirm that the princess would take part, and the Army did not ask royal officials for approval before publishing the notice, which it later removed.

Yesterday, friends declined to discuss Catherine’s reasons for her absence, wary of encouraging speculation about her health.

But one did tell me: ‘Catherine knows she should not overdo things. She is grateful that her recovery has gone well, but things have not been plain sailing.

‘She had a busy day at Trooping the Colour at the weekend and, if she’s feeling slightly under the weather, it’s better that she takes it easy.’

Given Catherine’s importance to the Royal Family, that’s a sentiment that all monarchists would agree with.

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