Prince Charles, heir to the British throne, has visited his father Prince Philip in hospital, said a Reuters photographer at the facility.
On Saturday (local time), Charles entered through the back of the London hospital where 99-year-old Philip, the husband of Queen Elizabeth, has been staying as a precautionary measure.
Charles was at the hospital for just over half-an-hour before departing.
Philip, the Duke of Edinburgh, was admitted to hospital on Tuesday after feeling ill with an ailment unrelated to COVID-19.
He is expected to remain in hospital until early next week, a royal source said on Friday, adding that doctors were acting out of an abundance of caution and the Duke remained in good spirits.
Both Philip and the Queen, 94, received their first dose of the COVID-19 vaccine in January.
Philip also spent four nights at the same hospital in 2019 while being treated for a pre-existing condition.
Philip is now rarely seen in public. He stepped down from official engagements in August 2017 after completing more than 22,000 solo events and thousands more alongside the Queen.
A former naval officer renowned for his sometimes brusque manner and humour, Philip married Elizabeth in 1947, five years before she became Queen. He is now by far the longest-serving consort of any British monarch.
In an apparent tribute to the Duke, one of his granddaughters, Princess Eugenie, and her husband Jack Brooksbank said on Saturday they had named their first son August Philip Hawke Brooksbank.