The King has recorded a video message to be played in Westminster Abbey next Monday.
The Queen and the Prince of Wales are to attend the annual Commonwealth Day service next week, joined by remaining members of the slimmed-down monarchy.
With the King and the Princess of Wales away from public-facing duties, the rest of the institution’s working royals will gather in force at Westminster Abbey on Monday.
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.
A pre-recorded video message from Head of the Commonwealth, Charles, who is undergoing treatment for cancer, will be played during the service this year, Buckingham Palace said.
The annual celebration will draw on the theme of resilience – at a time when the royal family has faced a barrage of health troubles.
But 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 event – a key part of the royal calendar – alongside Camilla comes after he pulled out of attending his godfather King Constantine of Greece’s memorial service last week at the last minute due to an undisclosed personal matter.
Camilla will carry out a second engagement afterwards when she attends the annual Commonwealth Day Reception at Marlborough House in London, meeting foreign ministers, UK parliamentarians and High Commissioners.
Some 2,000 guests will head to the Abbey in central London in honour of the Commonwealth network of 56 countries.
The congregation will include Commonwealth Secretary-General Patricia Scotland, the president of Malta, foreign ministers, high commissioners, senior politicians and dignitaries from across the Commonwealth, faith leaders, and school children 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 specialises in creating alternatives to plastic.
Writer Sir Ben Okri will read a specially commissioned poem for the 75th anniversary of the Commonwealth.
