The Duke of Cambridge and Prince Harry have visited their grandfather, the Duke of Edinburgh, in hospital.
Clarence House said the princes paid a "short private visit" to Prince Philip.
He was admitted to London's King Edward VII hospital with a bladder infection on Monday, and could not join the Queen at the final Diamond Jubilee events to mark her 60 years on the throne.
It is not yet known whether the duke will be discharged in time for his 91st birthday on Sunday.
The princes arrived at about 16:45 BST and left after some 20 minutes without saying anything to reporters outside.
Prince William visited his grandfather while on leave from his RAF search and rescue duties.
On Thursday, it was announced that he had qualified as a Royal Air Force search and rescue captain, after passing his tests on 29 May.
Meanwhile, his brother Prince Harry - a captain in the Army - has spent this week on pre-deployment training at Wattisham Airfield in Suffolk.
Earlier this week, Buckingham Palace said the duke's condition had "improved considerably" but he was likely to stay in the central London hospital for a few days.
The spokesman added that he was being treated with antibiotics and was in "good spirits".
Prince 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 as part of Jubilee celebrations.
The Queen and the Duke of York visited Prince Philip on Wednesday. Prince Andrew said that his father was "mending very well".
The duke had appeared to be in good health when he accompanied the Queen on Sunday on the royal barge the Spirit of Chartwell, which formed part of the rain-drenched Jubilee river pageant.