King Charles hailed the work of the Commonwealth in bringing countries together as he and Britain's senior royals gathered at London's Westminster Abbey on Monday for annual celebrations for the international organisation he leads.

Last year, both the 76-year-old king and his daughter-in-law Kate, the Princess of Wales, missed the Commonwealth Day service as they underwent treatment for cancer but both were present for Monday's occasion, one of the major royal events of the year.

In his annual message to the Commonwealth, a voluntary association of 56 nations, mostly former British colonies, Charles spoke of its success in bringing nations together and addressing environmental challenges, a cause he has championed for more than 50 years.

"In these uncertain times, where it is all too easy to believe that our differences are problems instead of a source of strength and an opportunity for learning, the Commonwealth’s remarkable collection of nations and peoples come together in the spirit of support and, crucially, friendship," he said.

"As we mark this Commonwealth Day together, there is no more important task than to restore the disrupted harmony of our entire planet. For the sake of our younger generations’ threatened future, I can only hope that the Commonwealth will continue its vital work to restore that harmony."

In what Charles said was an innovative way to mark the occasion, he has also revealed a playlist of 17 songs from Commonwealth artists which he said had brought him joy, were significant to him or made him want to dance.

'The King's Music Room', which is being aired on Apple Music's global radio stations, featured Jamaican reggae legend Bob Marley, Canadian jazz singer Michael Buble, Australian pop star Kylie Minogue, and U.S. singer-songwriter Beyonce, who Charles said was so exceptional he could not resist including her music.

His final choice was veteran U.S. singer Diana Ross's "Upside Down", which he said was a particular personal favourite.

"When I was much younger, it was absolutely impossible not to get up and dance when it was played," he said. "So, I wonder if I can still just manage it...?"