The Duke of Edinburgh faces spending his 91st birthday in hospital, where he is recovering from a bladder infection.
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.
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 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". Philip has also been visited by the Earl and Countess of Wessex and their children, Louise and James.
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.
Before the heart scare he had been fit and apparently healthy, and led the active life of a man of younger years.
In March this year his grandson Prince Harry said the operation had given him "a new spurt of life".
