The Duke of Edinburgh has undergone a successful procedure for a pre-existing heart condition, Buckingham Palace said.
Prince Philip was treated at St Bartholomew's Hospital on Wednesday.
The Palace said the duke, 99, would remain in hospital for treatment, rest and recuperation for a number of days.
Philip, the nation's longest-serving consort, has spent 16 nights in hospital - his longest ever stay.
He was moved by ambulance to St Bartholomew's Hospital in the City of London on Monday for testing for a pre-existing heart condition and treatment for an infection.
Buckingham Palace said in a statement: "The Duke of Edinburgh yesterday underwent a successful procedure for a pre-existing heart condition at St Bartholomew's Hospital.
"His Royal Highness will remain in hospital for treatment, rest and recuperation for a number of days".
Philip was was initially admitted to the private King Edward VII's Hospital on February 16 as a precautionary measure after feeling unwell, before being moved to St Bartholomew's on Monday.
Keep up to date with news in your area by adding your postcode below
Four days later, he was visited by his eldest son the Prince of Wales, who made a 200-mile round trip and stayed for around 30 minutes.
The duke has spent most of lockdown residing at Windsor Castle with the Queen for their safety, alongside a reduced household of staff dubbed HMS Bubble.
