Kate and William's foundation holds charity event in Windsor as guests left surprised by Prince of Wales

Prince William, 42, popped in to say hello to the visitors, according to the Director of Strategic Communications and Engagement at Wales Air Ambulance Charity.

The director, Steven Stokes, wrote on LinkedIn: "It’s not every day you get to see the inner rooms of Windsor Castle (no pictures allowed on the inside).

"It’s not every day you get to present in a private room in Windsor Castle.

"It’s not every day that HRH The Prince of Wales pops his head in to say hello.

"Thank you to the Royal Foundation for the invitation to present the work of the Wales Air Ambulance Charity, one of five charities invited to do so.

"The key points I focused on were the exceptional work of the staff, volunteers, medics, pilots and engineers, the valuable partnerships that we have with fellow emergency services, and the incredible generosity of the people of Wales that make our lifesaving service possible."

Stokes's encounter with William came just two days after the Prince of Wales travelled to Edinburgh for the Order of the Thistle service at St Giles' Cathedral.

The father-of-three travelled to Scotland to support Queen Camilla and Prince Edward, who were becoming members of the Order of the Thistle.

William made the trip without Princess Kate, 42, who is currently undergoing chemotherapy.

This also would explain why the Princess of Wales did not join her husband at the charity event on Friday.

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Last month, Kate announced she would attend Trooping the Colour, her first official public sighting in six months.

In a written statement, the mother-of-three said: "My treatment is ongoing and will be for a few more months.

"On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home.

"I’m looking forward to attending The King’s Birthday Parade this weekend with my family and hope to join a few public engagements over the summer, but equally knowing I am not out of the woods yet."