Britain's Prince Charles visited his father Philip in hospital on Saturday as the 99-year-old husband of Queen Elizabeth II remains in hospital as a precaution after feeling ill earlier in the week.

Charles, the first-born heir to the throne, was seen entering the rear entrance of the King Edward VII hospital in central London, with a face mask on.

Prince Philip, who turns 100 on June 10, was admitted on Tuesday. Sources say his admission was not coronavirus-related, and that Philip remains in good spirits.

The duke of Edinburgh, as he is formally known, is expected to remain in hospital into next week for observation and rest.

He was able to walk into the hospital unaided after arriving by private transport rather than ambulance, a source said at the time.

Both Philip and the queen received first doses of a Covid-19 vaccine in January.

Charles paid his hospital visit the day after Buckingham Palace confirmed that his younger son Prince Harry and his American wife Meghan had permanently quit as working royals.

Queen Elizabeth ordered the Duke and Duchess of Sussex to relinquish their honorary titles and patronages, a year after they stepped back from frontline duties. They now live in California.