Prince Philip remains in hospital after being admitted to King Edward VII Hospital in London on Friday.
Buckingham Palace said the Duke of Edinburgh, 98, was admitted for "observation and treatment in relation to a pre-existing condition".
He is reportedly in good spirits and has been laughing and joking with hospital staff.
In a statement on Friday, the palace said: “The Duke of Edinburgh travelled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
“The admission is a precautionary measure, on the advice of His Royal Highness's doctor.”
While some reports state that Philip walked into the hospital, the Daily Mail reports that the Duke was flown by helicopter from Norfolk to London, fuelling concerns over his health.
According to the publication, the prince has been "battling a flu-like illness for weeks" and suffered a fall before being rushed to hospital.
Speaking about the Duke’s health in recent months, one source said: “The Duke has had a bad turn recently and has been looking much frailer of late.
“He's a remarkable man who has always prided himself on how robust he is, but he is six months off his 99th birthday and age takes its toll.
“He hasn't been looking terribly well of late.”