Prince Charles has visited his father, the Duke of Edinburgh, at King Edward VII's Hospital.
Prince Phillip was admitted to the central London hospital earlier this week as a precautionary measure.
Charles arrived at around 15:20 on Saturday afternoon and left about 30 minutes later.
He is the first member of the Royal family to visit Philip during his four-night stay at the medical centre.
The Duke, who celebrated his 99th birthday in June 2020, is in hospital for an undisclosed reason, although it is not coronavirus-related.
On Friday, sources told PA news agency the Duke is "likely to remain in hospital for observation and rest over the weekend and into next week".
"As we have said previously, the doctor is acting with an abundance of caution. The duke remains in good spirits."
Prince Philip received a coronavirus jab last month.
He also gave his approval to a motto adopted by HMS Magpie, one of the Royal Navy's smallest ships, in January.
Last year, he handed over his role as Colonel-in-Chief of The Rifles to the Duchess of Cornwall in a rare public appearance.
Cover image: The Prince of Wales during a visit to the Queen Elizabeth Hospital in Birmingham (Picture: PA).