Charles III turns 75 on Tuesday, showing no sign of a let-up in activity just over a year since he became king after the death of his mother Queen Elizabeth II.

Halfway through his eighth decade, the British head of state is due to spend the day carrying out public engagements followed by a private dinner at his London residence.

The lifelong environmentalist will use the day to highlight causes close to his heart, including a visit to a surplus food distribution center with his wife Queen Camilla.

The visit will see him officially launch the Coronation Food Project, an initiative aimed at tackling food poverty by redistributing food that would otherwise end up in landfill.

Charles will also host a reception at Buckingham Palace for 400 nurses and midwives as part of this year's 75th anniversary celebrations for the state-run National Health Service (NHS).

The NHS choir will treat him to a surprise birthday song, and gun salutes will also sound in London and across the UK, Buckingham Palace announced.

Cake and a sing-song

Camilla, 76, once revealed that the famously workaholic king is particularly hard to buy gifts for.

"I will tell you that he is the most difficult person in the world to buy a present for… So he likes to make a list of things that he wants so you get it exactly right," she said.

He likes "a cake and a bit of a sing-song", she said, adding however that it was often difficult to get him to take a break.

The evening celebration will be attended by close family and friends, although his estranged younger son Harry will be missing.

A spokesperson for Harry and his American wife Meghan rebutted reports they had turned down an invitation saying there had been "no contact regarding an invitation to His Majesty's upcoming birthday".

Harry, 39, and Meghan, 42, quit royal duties in 2020 and relocated to California.