Britain's King Charles III on Monday marked the first anniversary of his coronation following a difficult year that has seen a cancer diagnosis and continuing tensions with his estranged son Prince Harry.

To celebrate the occasion a 41-gun salute rang out from Green Park next to Buckingham Palace at noon (1100 GMT), followed an hour later with 62 volleys fired from another central London location.

No official events, however, were planned with the milestone coming less than a week after King Charles resumed public duties since his diagnosis.

He visited fellow cancer patients at a London hospital on Tuesday after doctors said they were "very encouraged" by his progress.

The 75-year-old monarch, who has not disclosed the type of cancer he has, was also reportedly keen to mark the day with minimal "fuss" given the link between his accession and the death of his mother Queen Elizabeth II.

The Archbishop of Canterbury Justin Welby, who crowned King Charles a year ago at Westminster Abbey, praised his "sense of duty".

"The past year has presented the king with some great personal challenges. But I have been struck by his continued sense of duty, having recently returned to royal engagements following treatment," he said.

"His openness in sharing his condition has been characteristic of his willingness to help and support others," he added.

The anniversary comes as an Ipsos poll for the Daily Mail saw King Charles's personal popularity rating rise slightly to 56 percent.

His son and heir Prince William and his wife Catherine, the Princess of Wales, were the most popular members of the royal family on 69 percent each, up 8-10 percent since last April.

- Double blow -

Catherine, 42, also known as Kate, announced her own shock cancer diagnosis in late March following abdominal surgery which had already forced her to withdraw from public life.

The double blow of both Charles's and Catherine's ill health has left the royal family facing a shortage of frontline royals, with William also stepping back for a while to help care for his wife and family.

Queen Camilla, 76, stepped in to take over many of her husband's engagements while King Charles's sister Princess Anne and his youngest brother Prince Edward have also taken on more prominent roles.

But King Charles and Catherine's absence was felt even more keenly as the king's younger son Harry is no longer a working royal.

Harry dramatically relocated to North America in 2020 with his wife Meghan Markle.

Harry will return to the UK this week to attend a ceremony marking the 10th anniversary of his Invictus Games for disabled military veterans.

The domestic PA agency reported that he was "likely to be reunited with (King) Charles" at some point during his visit.

A demonstration organised by the anti-monarchy group Republic on Sunday, meanwhile, attracted around 100 people in Trafalgar Square.

Protesters shouted "abdicate, abdicate" and two large banners reading "abolish the monarchy" and "change (the) country for good" were hung in front of an art gallery.

Last year's coronation saw tens of thousands of people line the streets of London and street parties held up and down the country.

Television images of the historic ceremony at Westminster Abbey was beamed around the world.