Prince William says the Duke of Edinburgh is "ok" and doctors are "keeping an eye on him" in hospital.
The Duke of Cambridge's comments came as he met NHS staff and volunteers at a vaccination centre in King's Lynn, Norfolk.
Prince Philip, 99, has spent a sixth night at King Edward VII Hospital in central London after being admitted last Tuesday "as a precautionary measure".
A royal source said the reason for the hospital stay was not coronavirus-related.
On Saturday, the Duke of Edinburgh was visited by his son the Prince of Wales, and the pair spent about 30 minutes together.
Sky News understands Prince Charles travelled from Highgrove in Gloucestershire and was visiting because the duke had been in hospital a little longer than expected.
On Friday, it was reported that the Duke of Edinburgh was likely to remain in hospital until later this week for rest and observation, although he was said to be in "good spirits".