The King has pledged to continue to serve the Commonwealth “to the best of my ability”, in his annual address to the family of nations.

Since his cancer diagnosis was announced on February 5, he has postponed all public-facing duties, but is continuing with behind-the-scenes work on his red boxes of state papers and holding some in-person meetings.

Charles is receiving regular treatment for an undisclosed form of cancer, and recorded a video message for Commonwealth Day, to be marked by a Westminster Abbey service later, which he will miss.

His address highlighted how “diversity” was the Commonwealth’s core “strength”, and the union of 56 member countries was a “precious source of strength, inspiration, and pride” to him.

“In recent weeks, I have been most deeply touched by your wonderfully kind and thoughtful good wishes for my health and, in return, can only continue to serve you, to the best of my ability, throughout the Commonwealth,” the King said in his video message, which will feature in the service.

He added: “My belief in our shared endeavours and in the potential of our people remains as sure and strong as it has ever been.

“I have no doubt that we will continue to support one another across the Commonwealth as, together, we continue this vital journey.”

The Queen and the Prince of Wales will attend the service with members of the royal family, but without the Princess of Wales, who is still recovering from abdominal surgery and has only been since once in public since her operation in January.

Camilla and William will be accompanied by the Duke and Duchess of Edinburgh, the Princess Royal, the Duke and Duchess of Gloucester and the Duke of Kent.

The annual celebration will draw on the theme of resilience, at a time when the royal family has faced a barrage of health troubles.

The King stressed in his address how unity was essential for the family of nations, saying: “I cannot say often enough that it is by coming together that we create the best chances to improve our world and the lives of people everywhere.”

His remarks also recognised the 75th anniversary of the Commonwealth, established in 1949 when eight countries, Australia, Britain, Ceylon, India, New Zealand, Pakistan, South Africa and Canada, signed the Declaration of London.

In past years some commentators have criticised the Commonwealth for its apparent weakness and ineffectiveness when dealing with member states who do not abide by the rules.

But supporters of the family of nations believe it allows countries to unify on common causes and ultimately, through unity, effect change.

Charles said: “As I have said before, the Commonwealth is like the wiring of a house, and its people, our energy and our ideas are the current that runs through those wires.

“Together and individually we are strengthened by sharing perspectives and experiences, and by offering and borrowing the myriad ways we have each tackled the challenges of our time.

“This is true both at the level of nations and, indeed, at the local level. We recognise today that our diversity is our greatest strength.”

He went on: “Wherever we live, we are united by the many challenges we face, whether it be climate change, the loss of nature, or the social and economic changes that new technologies are bringing.”

The service will focus on stories of environmental resilience and marine sustainability, through performances representing the five regions of the Commonwealth.

William’s appearance at the televised service, a key part of the royal calendar, alongside Camilla, comes after he pulled out of attending the recent memorial service for his godfather, King Constantine of Greece, at the last minute, because of an undisclosed personal matter.

Camilla will afterwards attend the annual Commonwealth Day Reception at Marlborough House in London, to meet foreign ministers, UK parliamentarians and high commissioners.

Among the 2,000 guests will be Commonwealth Secretary-General Baroness Scotland, the president of Malta, foreign ministers, high commissioners, senior politicians and dignitaries from across the Commonwealth, faith leaders and schoolchildren and young people from throughout the UK.

Reflections will be given by one of the winners of William’s Earthshot Prize, Notpla, a firm which produces seaweed-based packaging.

Writer Sir Ben Okri will read a specially commissioned poem for the 75th anniversary of the Commonwealth.