Charles Mountbatten-Windsor has visited his father, who remains hospitalised under undisclosed circumstances.
Mountbatten-Windsor spent around 30 minutes at King Edward VII's Hospital on Saturday.
Phillip Mountbatten was admitted to hospital on Tuesday as a precautionary measure after feeling unwell. He will likely remain there until next week, Buckingham Palace has said.
Mountbatten received the first dose of his Covid-19 vaccine in January, and his hospitalisation has been confirmed to be unrelated to Covid-19.
The details of Mountbatten's medical situation have not been disclosed, but the hospitalisation is said to have taken place out of "an abundance of caution," and Mountbatten is said to be in good spirits.
The Hospital's visitation policy allows visits from family only in end- of-life circumstances.
Buckingham Palace has yet to comment on the visit.