The King will fly to Australia and Samoa for the first long-haul overseas tour since his cancer diagnosis and join world leaders at a major Commonwealth summit.
Charles and Camilla will pass a number of milestone in their roles as monarch and consort during their southern hemisphere visit over nine days from October 18-26.
The health of the monarch will be a factor during the visit as the King has been receiving treatment as an out-patient for an undisclosed form of cancer since February.
The programme is likely to be designed to afford him time to rest and recover from the many hours he will spend flying during his trip.
The King is head of the Commonwealth and for the first time will preside over the gathering of presidents and prime ministers as head of state, during the event hosted by Samoa which will also be attended by Sir Keir Starmer.
Charles deputised for Queen Elizabeth II during the last Commonwealth Heads of Government Meeting (CHOGM) staged by Rwanda in 2022, but will now welcome the representatives as head of state.
His trip to Samoa has the status of a state visit and the royal couple will be joined by Foreign Secretary David Lammy, while the Australian tour is also significant as Charles is the nation’s head of state.
Australia’s prime minister Anthony Albanese has a long-held aim of holding a referendum on breaking ties with the British monarchy and his country becoming a republic.
But the plans were put on hold after Australians overwhelmingly rejected a plan to give greater political rights to Indigenous people in a referendum held last year.
Charles will meet colleagues Professor Georgina Long and Professor Richard Scolyer, named as Australians of the year 2024 in recognition of their pivotal work on melanoma, one of Australia’s most common cancers.
Their pioneering research in using the body’s immune system to attack cancer cells has been applied to treat Prof Scolyer, who was diagnosed with a brain tumour but a scan earlier this year showed no recurrence of the cancer, according to reports.
Other highlights of the Australian leg will see the royal couple spending time in the capital Canberra meeting leading figures and paying their respects to the country’s fallen, while in western Sydney they will attend a community barbecue – a staple of Australian culture.
Royal travel for the Australian leg of the trip will be funded by its government as it is a realm, a country where Charles is head of state, while the state visit to Samoa will be paid for by the UK in the normal way.
In Samoa, the programme will follow a similar theme to Australia with the King attending events around sustainability and biodiversity, while the Queen will focus on her interests of literacy and domestic violence and sexual abuse.
A Buckingham palace spokesman said: “We’ve had to, as you would with any visit, think about how you can ensure their Majesty’s energies are preserved to be at their best.”
He added: “There had been some hope earlier in the year for their Majesties to be able to visit New Zealand. On doctor’s advice, and in close consultation with the Australian and New Zealand governments, that wasn’t able to take place.
“We’ve had to make some difficult decisions about the programme with the Australian government, about where their Majesties can get to.”