The Duke of Edinburgh is progressing satisfactorily after an exploratory operation, Buckingham Palace says has said.
The Queen's husband, who will be 92 on Monday, was admitted to the London Clinic on Thursday and is expected to stay in hospital for about two weeks.
Buckingham Palace said on Thursday the hospital admission for surgery on his abdomen was pre-arranged and not an emergency.
Prince Philip was said to be comfortable and the treatment went as planned, the BBC reports.
The palace said the results from the operation would now be analysed.
The Queen continued with engagements, officially opening the BBC's rebuilt Broadcasting House in central London, and will spend the weekend at Windsor.
Prince Philip was admitted straight after a Buckingham Palace garden party in the afternoon where guests said he showed no signs of being unwell.
In the past week, he has been having unannounced "abdominal investigations" at the hospital, where police officers have been standing guard outside.