A visit to Australia by King Charles is in doubt after the monarch was diagnosed with cancer.

Buckingham Palace on Tuesday revealed Charles, 75, had begun regular treatment and would postpone public duties.

The king was expected to tour Australia late this year accompanied by Queen Camilla. It would have been the couple’s first visit since 2018.

The Nine newspapers reported last year that King Charles was planning to visit Australia and New Zealand in October 2024.

The royal visit was to coincide with a broader trip to the region for the Commonwealth heads of government meeting (Chogm) in Samoa.

The Australian Monarchist League said on Tuesday the trip was “relatively locked-in”.

“The palace had indicated a desire to visit Australia in association with that visit to Samoa, so it was something that many of us were looking forward to,” the league’s chairman, Eric Abetz, told ABC TV.

“But now we will have to wait and see what the medical advice is as to whether the king is able to go to Chogm let alone to Australia as well.”

Abetz said the best wishes of all Australians were with the king and his family.

This year’s Chogm will focus on climate change – an issue on which King Charles has been outspoken. Addressing Cop28 last year, he called for the UN summit to be a “critical turning point” in addressing the “frightening experiment” being undertaken on the natural world.

“Records are now being broken so often that we are perhaps becoming immune to what they are really telling us,” Charles said.

“We need to pause to process what this actually means: we are taking the natural world outside balanced norms and limits, and into dangerous, uncharted territory.”

The Racing New South Wales chairman, Russell Balding, said last year he was “quietly confident” King Charles would attend Everest Day at the Royal Randwick racecourse in Sydney on 19 October.

He told Racing Confidential the monarch was keen to attend the meeting and watch a Group 1 race named in his honour – the 1,600-metre King Charles III Stakes.

“Following our private meeting with the King at Royal Ascot, we’re quietly confident of him attending the race next year,” Balding said at the time.

A royal visit to Australia by King Charles had not been formally confirmed.

He last visited Australia in 2018 with Camilla for the Gold Coast Commonwealth Games where he met crowds of well-wishers at the botanic gardens in Brisbane.

Charles’s heir, Prince William, and his wife, Kate, last visited Australia in 2014 with baby Prince George. There were reports the royal couple would visit bushfire-hit regions of the country in 2020 before the Covid pandemic hit.

The Australian prime minister, Anthony Albanese, said the news of Charles’s diagnosis was difficult.

“All Australians will be sending their best wishes to King Charles for a speedy recovery,” he told reporters in Canberra on Tuesday.

“We want to see his majesty return to full duties as soon as possible. We have invited King Charles to visit Australia and we sincerely hope that is able to occur.”

The federal opposition leader, Peter Dutton, on Tuesday urged men not to delay visiting their doctor.

“We wish every best wish to King Charles … he’s a stoic individual,” he said.

“There’s a message the palace wants to get out as well. Make sure you go and have a checkup because early detection is important.”

The king’s cancer was detected during recent treatment for an enlarged prostate.