King Charles is like a "caged lion" and frustrated by his cancer diagnosis because he's got so much more to achieve, an insider has claimed.

His Majesty - who is well-known for his love of letter writing - has continued to pen notes to his nearest and dearest in the weeks and months following the devastating news. One recipient says he has spoken of his "somewhat battered health", but that the letters are still positive and "full of light".

The correspondences describe his "determination" to triumph over the condition. One pen pal says they're "full of exclamation marks" especially when he writes about his recovery.

King Charles III and Queen Camilla arriving for a visit to UCL, London at the end of April (
Image:
PA)

Speaking about his three-month hiatus from public duties, one friendly source told The Times: "He has been frustrated, because there is still so much he wants to achieve. He holds himself to very high standards of public service and genuinely feels he’s letting people and organisations down if he’s not out there doing all those public bits of his formal role.” Another friend chimed in, saying: "He’s a bloody caged lion, driving everyone round the twist if he’s stuck at home."

Just days after returning to public duties this week, the king made a surprise visit to one of his late mother's favourite events - where he received a warm welcome from his niece Zara Tindall.

The monarch was spotted at the Royal Windsor Horse Show yesterday, which is taking place close to his Windsor Castle residence. The annual event was one of the late Queen's favourites, with her attending almost every year since she was a young girl until her death in 2022. Today Charles beamed as he braved the wet weather to tour some of the showgrounds, where some of his horses in action.

King Charles and Zara Tindall greeting each other yesterday at the Royal Windsor Horse Show (
Image:
Getty Images)

Also attending the event today was Princess Anne's daughter Zara, a champion equestrian, who couldn't wait to greet her uncle the King, and the pair were spotted having a sweet moment as they greeted each other warmly at the show.

The King met his niece overlooking the Royal Windsor Endurance course, in Windsor's Great Park, which takes riders along landmarks like the famous Long Walk. Among the royal group were Lord Soames, a long-time friend of the King, who served as his equerry when Charles was the Prince of Wales, comic Rory Bremner, former racing driver Jackie Stewart and ex-Defence Secretary Ben Wallace. Charles was also pictured chatting to Simon Brooks-Ward, director of the Royal Windsor Horse Show, who knew the late Queen well.

The King's surprise visit to event comes just days after spoke of his "shock" at being diagnosed with cancer as he met fellow patients on his return to public-facing duties. Charles candidly described the experience of being told he had the disease when he visited University College Hospital Macmillan Cancer Centre in central London, to learn about the latest treatments and chat to staff providing the pioneering care.