Buckingham Palace said this evening that Duke of Edinburgh Prince Philip was "progressing satisfactorily", following a planned exploratory operation in hospital.
The Duke, who will be 92 on Monday, had surgery on his abdomen under general anaesthetic and the results will now be analysed.
He was admitted to the London Clinic on Thursday evening after appearing at a garden party.
The Palace said in a short statement: "His Royal Highness the Duke of Edinburgh has had an exploratory operation following abdominal investigations. The results will now be analysed.
"At this early stage he is progressing satisfactorily.
"Further updates will continue to be issued when appropriate."
This evening the Palace said the Duke was 'comfortable'.
It is thought that the operation on his abdomen did not involve his heart or bladder, organs the Duke has experienced problems with before.
The Queen has been kept fully informed of her husband's progress and is spending the weekend as planned at her Berkshire home, Windsor Castle.
The analysis report on the "results" of the operation are expected to be completed next week.
Peter Phillips, the Duke's eldest grandson, said the family were being informed about his grandfather's progress, and he would not want them to stop.
He told BBC News: "Obviously our thoughts and prayers are with him, we know that he's in good hands and we've been kept up to date in terms of what's happening.
"But he's one of those people who wouldn't want you to stop just because he's gone into hospital."
Details about the Duke's original symptoms that prompted the exploratory operation have not been released by Buckingham Palace.
But earlier today when the Queen opened the BBC's revamped Broadcasting House in London she was asked how her husband was by Radio 4 Today presenter John Humphrys and replied: "I don't know."
The broadcaster added that the Duke looked well during his appearance at yesterday's Buckingham Palace garden party, and the monarch stated: "That's because he's not ill".