Prince Philip, the Duke of Edinburgh, is to remain in hospital until at least early next week.
The 99-year-old was taken to the private King Edward VII hospital in Marylebone, London, on Tuesday night after falling unwell.
Details of Philip's condition have not yet been disclosed, although it is not connected to coronavirus and he was well enough to walk into hospital when he was admitted.
A source told The Sun: "Following consultation with his doctor The Duke of Edinburgh 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."
The Queen is said to be remaining at Windsor Castle while her husband convalesces.