The Queen has visited the Duke of Edinburgh in hospital, where he is said to be "comfortable and in good spirits" on his 92nd birthday.
Her Majesty's 25-minute visit to the London Clinic in Harley Street, where Prince Philip underwent exploratory surgery on his abdomen on Friday, came as Buckingham Palace gave an update on his condition.
In a statement, palace officials said he is likely to remain in hospital for up to two weeks and will then take a "period of convalescence of approximately two months".
The Queen visited the hospital after returning to Buckingham Palace following a weekend at Windsor Castle, where she received regular updates about her husband's health.
Her Majesty was the second member of the Royal Family to visit Prince Philip on Monday.
Earlier, Prince Edward spent around 20 minutes at the hospital. Asked by reporters how his father was, he replied: "Well, thank you."
Gun salutes were fired around the country to mark Prince Philip's birthday.
The King's Troop Royal Horse Artillery fired a 41-gun royal salute in Green Park, central London, at noon.
There were also salutes at Edinburgh Castle, in Cardiff and at Hillsborough Castle in Northern Ireland.
Prince Philip received birthday greetings on Twitter from Prime Minister David Cameron.
He wrote: "Wishing the Duke of Edinburgh a very happy 92nd birthday as he recovers in hospital today."
The prince was admitted to hospital for the pre-arranged operation on Thursday, following a garden party at the Palace.
Analysis of the results are expected to be completed this week.
Details of the original symptoms behind his admission have not been released, but it is thought the procedure did not involve his heart or bladder, with which he has experienced problems before.
The prince was admitted to hospital twice last year because of a bladder infection that forced him to miss the last of the Queen's Diamond Jubilee celebrations.
He also suffered a blocked coronary artery the previous Christmas that required surgery.
Despite being admitted to hospital on several occasions in recent years, the prince, known for his active lifestyle, has maintained a busy schedule, and in April visited Canada for a "private working visit".
He was at the Queen's side at Westminster Abbey last Tuesday for the 60th anniversary of her coronation and on Wednesday toured a Victorian steamship, the SS Robin, in London's East End.
On Thursday, ahead of the garden party, the Queen invested Prince Philip with New Zealand's highest honour, making him an additional member of the Order of New Zealand to mark the Diamond Jubilee.