Britain's King Charles III officially handed over the role of Colonel-in-Chief of the Army Air Corps to Prince William on Monday, in a rare joint engagement with his elder son and heir.

The head of state joined William at the Army Aviation Centre in Middle Wallop, southern England, appointing him colonel-in-chief of his younger son Prince Harry's old regiment.

The 75-year-old monarch's decision to hand the role, which he held for 32 years, to William, 41, was seen as a snub to Harry when it was announced last year.

Harry, 39, is a former army captain and served as an Apache helicopter pilot with the Army Air Corps in Afghanistan.

But he has become estranged from his father and brother since leaving the royal family in early 2020 and moving to the United States with his American wife, Meghan.

The king, who announced he had cancer in February and is receiving treatment, said the event was bittersweet, as it was a "great joy" to attend but sad to hand over the baton after 32 years.

"I do hope you'll go from strength to strength in the future with the Prince of Wales as your new colonel-in-chief," he told the regiment in a speech.

"The great thing is he's a very good pilot indeed. So that's encouraging," said the king.

William worked as a Royal Air Force search and rescue pilot and also flew for a civilian air ambulance unit before taking up full-time royal duties.

His appointment came just days after Harry visited London but did not meet his father due to what he said was the king's "full programme".

The lack of meeting -- even for a short time -- was interpreted as a blow to Harry, who then flew on to Nigeria and a three-day tour with Meghan.

The Duke of Sussex, as Harry is also known, did not meet William either, who was scheduled for a visit to Cornwall in southwest England.

Harry's relationship with his father has been strained by his revelations and criticisms of royal life aired in a television interview, a Netflix documentary and his autobiography "Spare".

A possible end to four years of rancour and score settling was previously seen as on the cards in February, when Harry jetted in from California to briefly see his father.

The king has recently resumed public engagements after doctors said they were "very encouraged" by the progress of his treatment.