"Unfortunately, The Prince of Wales is no longer able to attend the King Constantine Memorial Service this morning due to a personal matter," Kensington Palace said.
The palace declined to give any further details, but it comes after his wife Kate underwent abdominal surgery last month.
However, a royal source said Kate was continuing to do well.
(Reporting by Sachin Ravikumar and Michael Holden, editing by Sarah Young and Kate Holton)