Britain's Prince Philip, the 99-year-old husband of Queen Elizabeth II, left a London hospital on Tuesday after a four-week stay for treatment for an infection and to have a heart procedure.

Philip, the Duke of Edinburgh, was admitted to the private King Edward VII's Hospital on February 16 after he felt unwell and was given treatment for an unspecified, but not COVID-19-related, illness.

He was transferred briefly at the start of this month to a specialist cardiac center at another London hospital, where he underwent a successful procedure for a pre-existing heart condition.

A Reuters witness said the duke left the King Edward hospital shortly after 10.30 GMT on Tuesday. He was taken to a waiting car in a wheelchair and returned to Windsor Castle, the royal residence west of London, waving to those outside as he arrived.

"His Royal Highness wishes to thank all the medical staff who looked after him at both King Edward VII's Hospital and St Bartholomew's Hospital, and everyone who has sent their good wishes," Buckingham Palace said in a statement.