Kate, Britain's Princess of Wales, has undergone successful planned abdominal surgery and will be in hospital for up to two weeks, her Kensington Palace office said on Wednesday, according to the Reuters news agency.
Kate, 42, the wife of heir to the throne Prince William, had been admitted to the private London Clinic on Tuesday where she is expected to remain for 10 to 14 days before returning home to recover, the palace added.
No details of the surgery were given, but a palace spokesman said the condition was non-cancerous.
"The Princess of Wales appreciates the interest this statement will generate," the statement said. "She hopes that the public will understand her desire to maintain as much normality for her children as possible; and her wish that her personal medical information remains private."
Kensington Palace said it would only provide updates on Kate's progress when there was "significant new information to share". It added that, on medical advice, she was unlikely to return to public duties until after Easter.
Less than two hours after that unexpected announcement, Buckingham Palace revealed that King Charles, 75, would also attend hospital next week for a "corrective procedure" but said his condition was benign.
"In common with thousands of men each year, the king has sought treatment for an enlarged prostate," the palace said in a statement.