Doctors are treating Prince Philip, 99, for an infection. They will also undertake testing and observation for a pre-existing heart condition, Buckingham Palace said.
The duke is responding to treatment and "remains comfortable", it added.
He is expected to remain in hospital until at least the end of the week.
Philip, who turns 100 in June, was taken to King Edward VII's hospital in London on 16 February as a precaution after feeling unwell. A palace source at the time said the duke had walked into the hospital unaided.