LONDON -- Prince William has pulled out of attending a memorial service at Windsor Castle for his godfather, the late King Constantine of Greece, because of an unspecified personal reason, his office said Tuesday.

"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, 42, underwent abdominal surgery last month. Officials have previously said Kate was making good progress in her recovery and following the announcement on Tuesday, a royal source said Kate was continuing to do well.

It is rare, but not unheard of, for the royals to pull out of an engagement. William had called his family to let them know he would be unable to attend, the source said.

Following Kate's operation, William, the 41-year-old heir to the throne, postponed official duties to care for their three children, before resuming engagements earlier this month. His wife is not due to return to public events until after Easter.

His absence comes as his father King Charles, 75, is also unable to perform his public role as he undergoes treatment for an unspecified form of cancer.

Another royal source said there was no connection between William's personal matter and the king's health.

In Charles' absence, his wife Camilla has become the most senior royal performing engagements.

She was joined by other senior royals at the service at Windsor Castle for the late King Constantine, who died aged 82 in January last year and was a relation of William's grandfather.