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 made a "short private visit" to him, said Clarence House.
The Duke was admitted to the King Edward VII Hospital in central London on Monday and faces spending his 91st birthday there if he is not discharged by Sunday.
William visited his grandfather while on leave from his RAF Search and Rescue duties. His brother Harry, meanwhile, has been spending the week since the Diamond Jubilee celebrations on pre-deployment training at Wattisham in Suffolk.
The princes, dressed in jeans and V-neck jumpers, left the hospital after 20 minutes without commenting. They were whisked away in a silver Range Rover after their private visit.
Philip has already been visited by the Queen, the Duke of York, the Earl and Countess of Wessex and their children, Louise and James.
Andrew said on Wednesday that his father was "mending very well" but Buckingham Palace said they had no further update on his condition and did not know when he would be discharged.
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 the Sandringham Estate to mark her Diamond Jubilee.
It is hoped her husband will be by her side at her Norfolk retreat if he has been discharged in time. It is also hoped he will be able to accompany her to the East Midlands on Wednesday for a two-day visit as part of the Queen's jubilee tour of the UK.
The King's Troop Royal Horse Artillery will fire a salute in London's Hyde Park at midday next Monday to mark Philip's birthday, the Ministry of Defence said. The salute is fired every year to herald the occasion and the event is open to the public but not customarily attended by members of the Royal Family.