'Happy Father's Day Pa' message from Prince William to King Charles

The photo of William and Charles together in the gardens of Kensington Palace was taken on June 12 1984.

"Happy Father's Day, Pa," the social media post, personally signed by William, said.

The photo taken by the PA news agency shows an almost two-year-old William in short dungarees and a stripey blue top and the King in a grey double-breasted suit kicking a small football.

The Prince of Wales playing football in the garden of Kensington Palace, London, with his father King Charles III posted by the Prince of Wales on royal social media channels to mark Father's Day. "Happy Father's Day, Pa," the post, personally signed by William, said.

Later that same year William would gain a younger brother, with the Duke of Sussex born in September.

It comes after Charles's official birthday celebrations on Saturday, where the Princess of Wales made her first public appearance this year.

Kate looked relaxed during her first day in the spotlight following her cancer diagnosis for the traditional Trooping the Colour ceremony.

With the King also suffering from cancer, it was a rare joint public outing for the entire family after a period of uncertainty.

She rode to the event in a carriage with her children, while William, the Princess Royal, and the Duke of Edinburgh were on horseback.

The King also rode in a carriage with the Queen, a departure from last year because of his illness.

Kate and members of the royal family were cheered when they were first spotted in The Mall.

Both Kate and Charles have thanked the public for their support after going public with their illnesses, with Kate saying she had been "blown away" by the "kind messages" that had made a "world of difference to William and me and has helped us both through some of the harder times".