The Princess of Wales is set 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 months.
The palace did not disclose further details about her condition but it is understood it is not cancer-related.
A statement added Catherine, 42, wished to apologise for postponing planned engagements.
“Based on the current medical advice, she is unlikely to return to public duties until after Easter,” a statement said.
As is standard practice for senior royals, the palace has disclosed few details about the princess’ condition and will not provide a running commentary on her recovery.
But it is clear from the length of time Catherine is expected to stay in hospital and the tone of the statement issued by the palace that her medical condition is serious.
The surgery – which took place on Tuesday – was significant enough to keep her in hospital for up to two weeks and it is expected that recuperation could take as long as three months.
Many minor conditions can be treated and the patient sent home shortly after.
The princess had a full diary in December and there was no indication that she was unwell during public appearances.
A statement from the palace said the princess “appreciates the interest this statement will generate” and stressed she wants her personal medical information to remain private.
It continued: “She hopes that the public will understand her desire to maintain as much normality for her children as possible.”
Kensington Palace said it would only provide updates when there is significant new information to share.
She is recuperating at the London Clinic, near Regent’s Park in central London, which describes itself as the UK’s largest independent private hospital.
TV cameras and reporters were seen gathering near to the hospital after the announcement on Wednesday afternoon, with a number of police officers also discreetly dotted around the streets near the building.
The Prince of Wales will not be carrying out any official duties while his wife is in hospital.
Once discharged, she is expected to recover at home in Windsor, where the Waleses and their children, Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, now reside.
In the statement issued on Wednesday afternoon, Kensington Palace said: “Her Royal Highness The Princess of Wales was admitted to The London Clinic yesterday for planned abdominal surgery.
“The surgery was successful and it is expected that she will remain in hospital for ten to fourteen days, before returning home to continue her recovery.”