The Princess of Wales is to stay in hospital for up to a fortnight after undergoing abdominal surgery.
Kensington Palace said the procedure was planned and successful, but the princess is not expected to resume royal duties for several months.
The palace has not released further details about his condition, but it is understood it is not cancer-related.
Catherine, 42, wants to apologize for postponing planned engagements, a report said.
“Based on current medical advice, he is unlikely to return to public duties until after Easter,” a statement said.
As is standard practice for senior royals, the palace released few details about the princess's condition and would not provide a running commentary on her recovery.
But the length of Catherine's expected hospital stay and her condition are clear from the tone of the statement released by the palace.
The surgery – which took place on Tuesday – was significant enough to keep her in hospital for up to two weeks, and recovery is expected to take up to three months.
Many minor conditions can be treated and the patient can be sent home quickly.
The princess kept a full diary in December and showed no sign of being unwell during public appearances.
In a statement from the palace, the princess “appreciates the interest in making this report” and insisted that her personal medical information remain private.
It continued: “She hopes the public will understand her desire to maintain as much normality as possible for her children.”
Kensington Palace said it would only provide updates when there was significant new information to share.
He is recovering at the London Clinic, near Regent's Park in central London, which describes itself as the largest independent private hospital in England.
Television cameras and reporters could be seen gathering near the hospital after the announcement Wednesday afternoon, with several police officers discreetly swarming the streets near the building.
The Prince of Wales will not carry out any official duties while his wife is in hospital.
Once discharged, she is expected to recuperate at the home in Windsor where Wales and their children Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, now live.
In a statement released on Wednesday afternoon, Kensington Palace said: “Her Royal Highness The Princess of Wales was admitted to a London clinic yesterday for elective abdominal surgery.
“The surgery was successful and he is expected to be in the hospital for ten to fourteen days before returning home to recover.”