The Duke of Edinburgh had a successful operation for a heart condition, Buckingham Palace said on Thursday.
Prince Philip, 99, spent 16 nights in hospital after being admitted on February 16 as a precautionary measure.
He was treated for an infection at King Edward VII's Hospital in London before being taken by ambulance to St Bartholomew's on Monday for heart tests.
Buckingham Palace said the surgery took place on Wednesday.
"The Duke of Edinburgh yesterday underwent a successful procedure for a pre-existing heart condition at St Bartholomew's Hospital," it said.
"His royal highness will remain in hospital for treatment, rest and recuperation for a number of days."
The reason for the duke's initial admission to hospital was not disclosed but it is not believed to be related to coronavirus.
Prince Philip and Queen Elizabeth II received their first dose of a Covid-19 vaccine in January.
The queen, 94, is at Windsor Castle in England continuing her official duties by video.
The duke was visited in hospital by his son and heir to the throne Prince Charles.
Camilla, Duchess of Cornwall, said on Wednesday that the duke's condition had "slightly improved" but he still "hurts at moments".
"We keep our fingers crossed," she told a volunteer during a visit to a Covid-19 vaccination centre.
Prince Philip, who turns 100 in June, required hospital treatment for a bladder infection several times in the past decade.
At Christmas in 2011 he had an operation to clear a blocked artery in his heart after being taken to hospital with chest pains.
The duke's present hospital stay is his longest and comes at a difficult time for the UK royal family, which faces a rift with the Duke and Duchess of Sussex, whose tell-all interview with Oprah Winfrey will be broadcast in the US on Sunday.
In a second teaser clip of the interview, Meghan Markle accuses Buckingham Palace of "perpetuating falsehoods" about her and Prince Harry.
"I don't know how they could expect that after all of this time we would still just be silent, if there is an active role that 'The Firm' is playing in perpetuating falsehoods about us," she said.
"And if that comes with the risk of losing things ... there's a lot that's been lost already.”
The interview was filmed before aides accused Ms Markle of bullying during her time as a working royal.
In a highly unusual statement about the media reports, Buckingham Palace said it would investigate the claims.
