The Duke of Cambridge and Prince Harry have gone to see the Duke of Edinburgh in hospital, where he is recovering from a bladder infection.
Philip's grandsons were paying a "short private visit" to him, Clarence House said.
The Duke was admitted to the King Edward VII Hospital in London on Monday and faces spending his 91st birthday there tomorrow.
Philip had been planning to celebrate his birthday privately and is not due to attend his next scheduled engagement until Tuesday, when the Queen hosts a garden party at Sandringham.
The Palace said earlier this week that he was being treated with antibiotics and was in good spirits.
Last December, Philip underwent a successful procedure to clear a blocked coronary artery and was in hospital for four nights over Christmas.