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.

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.

They have since unleashed a barrage of criticisms of the royal family leading to strained relations with his father and a damaging rift between Harry and his older brother, heir to the throne Prince William.

Prince Charles Philip Arthur George was born on 14 November 1948 at Buckingham Palace, the first child of future Queen Elizabeth II and Prince Philip, the Duke of Edinburgh.

When he turned 70 in 2018, Charles joked that it was "alarming" and that he had acquired "all the scars that go with" his age.

Charles is marking his 75th birthday landmark in the same year that he was crowned and just a week after opening the UK parliament for the first time as sovereign. 

Like his mother, who died at the age of 96 in September 2022, Charles has maintained a busy diary of royal duties despite his advancing years.

But Ed Owens, a royal historian and author, told AFP that Charles had taken on a more active role on the international stage than the late sovereign.

Charles had adopted the role of a "kind of international lead diplomat of Great Britain" and the Commonwealth, said Owens, author of "After Elizabeth: Can the Monarchy Save Itself?"

He had also shown that he was happy to speak out about difficult issues linked to colonialism and the British Empire.

On a visit to Kenya earlier this month, Charles acknowledged there was "no excuse" for colonial-era abuses committed in the East African country.

"He's confronting some of those more problematic histories in a way that Elizabeth II never would have done," Owens added.