Britain's Prince Philip has had a successful procedure for a pre-existing heart condition, Buckingham Palace said Thursday.

Britain's Prince Philip, Duke of Edinburgh, reacts as he attends a parade in the role of Royal Marines' Captain General for the last time at Buckingham Palace in London, Britain on Aug. 2, 2017. Prince Philip, husband of Queen Elizabeth II, carries out his final solo public engagement Wednesday before he retires from royal duties. (Xinhua/Pool)

The palace said the procedure took place on Wednesday and the 99-year-old Duke of Edinburgh "will remain in hospital for treatment, rest and recuperation for a number of days."

Philip, who turns 100 in June, is being treated at St Bartholomew's Hospital in London for his heart condition. He was transferred Monday by ambulance from the King Edward VII's Hospital, where he was treated for an infection.

He has already spent 16 nights in hospital.

The exact reason for his initial admission was not disclosed, but it was not related to coronavirus, according to the Buckingham Palace.

Prince Charles visited his father in the hospital last month and spent about 30 minutes together.

Philip retired from public duties in 2017 and rarely appears in public. He has been staying with the Queen Elizabeth II at Windsor Castle since COVID-19 restrictions were first introduced in the country last year.

Last month, Buckingham Palace confirmed the Queen and Philip had both received their first dose of a coronavirus vaccine.

To bring life back to normal, countries such as Britain, China, Germany, Russia and the United States have been racing against time to roll out coronavirus vaccines.