The Duke of Cambridge and Prince Harry are visiting The Duke of Edinburgh in hospital, Clarence House said on Friday.
Prince Philip's grandsons were paying a "short private visit" to him, Clarence House said.
The Prince, who is recovering from a bladder infection, faces spending his 91st birthday in hospital. Buckingham Palace said he might not be discharged by Sunday despite the Duke of York's assurance this week that his father was on the mend.
Philip had been planning to celebrate privately and was 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 from the King Edward VII Hospital in central London by then.
The King's Troop Royal Horse Artillery is expected to fire a salute in London's Hyde Park 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.
Philip was forced to miss some of the jubilee celebrations this week after he was admitted to hospital on Monday.
He had spent the previous day braving the elements for the Thames River Pageant, which doctors suggested might have exacerbated his condition if he did not make enough visits to the toilet during it.
On Wednesday he was visited by the Queen and Andrew, who said his father was "mending very well".
The Palace said earlier this week that he was being treated with antibiotics and was in good spirits.
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