The Duke of Edinburgh has been transferred to St Bartholomew’s Hospital for continued treatment, after 13 nights at another London hospital.
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.
On Monday morning, someone was screened from journalists’ cameras by umbrellas as they got into an ambulance outside the hospital in Marylebone.
Philip was transferred about three miles (5km) to St Bartholomew’s Hospital, also known as Bart’s, in the City of London.
St Bart’s is a centre of excellence for cardiac care, its NHS site says.
The exact reason for Philip’s initial admission has not been disclosed, but his stay is not related to coronavirus.
Both the duke and the Queen, 94, received Covid-19 vaccinations last month.