Prince Philip, who was hospitalized last week, is being treated for an infection, according to Buckingham Palace.
The palace did not provide any further details about the infection or what type of treatment Philip is undergoing, saying only, "He is comfortable and responding to treatment but is not expected to leave hospital for several days."
The Duke of Edinburgh, who will turn 100 in June, was taken by car from Windsor, England, to the King Edward VII Hospital in London on Feb. 16 for what the palace described as a "precautionary measure" after Philip reported feeling unwell.
Philip's illness is not COVID-19-related, a royal source told ABC News.
Philip's oldest son, Prince Charles, visited his father in the hospital on Saturday and stayed for around 30 minutes. Visitors are only allowed at the hospital in “exceptional circumstances” because of the coronavirus pandemic, according to the hospital's website.
Charles, who was photographed entering the hospital wearing a face mask, is believed to be the only family member so far to have visited Philip in the hospital.
The duke's youngest child, Prince Edward, told Sky News on Tuesday that he had spoken with his father by phone.
"As far as I'm aware, well, I did speak to him the other day, so he's a lot better thank you very much indeed, and he's looking forward to getting out, which is the most positive thing," Edward said of Philip. "So we keep our fingers crossed."
Philip's grandson, Prince William, also spoke about his condition while visiting a vaccine center in Norfolk, telling longtime royal photographer Arthur Edwards that Philip is "OK," adding, "They're keeping an eye on him."
While Philip is hospitalized in London, Queen Elizabeth remains at Windsor Castle, where she has been staying with her husband for most of the coronavirus pandemic.
The queen and Philip celebrated their 73rd wedding anniversary in November.