Prince Philip, the Duke of Edinburgh, withdrew from an official engagement after falling ill - a day before the service to mark the Queen's 60-year reign.
The Duke of Edinburgh was due to join the Queen Elizabeth II at a gala reception last night, for the Royal National Institute of Blind People at St James's Palace.
However, he pulled out at the last minute as he was said to be "unwell", though no details of his illness have been released.
A Buckingham Palace spokeswoman said: "The Duke is unwell, but he will be at the service tomorrow."
Prince Phillip, 91, was treated twice last year for a bladder infection that forced him to miss part of the Queen's Diamond Jubilee celebrations.
The latest health-related episode will raise inevitable questions about the strains imposed on the Duke by his workload of royal engagements.
