Buckingham Palace says the Duke of Edinburgh is receiving treatment for an infection and is not expected to leave hospital for several days.
Prince Philip was admitted to London's King Edward the Seventh's Hospital last Tuesday as a precaution after “feeling unwell”.
The palace said the 99-year-old husband of Queen Elizabeth II is comfortable and responding to treatment.
Both the duke and the Queen received COVID-19 vaccinations last month.
Buckingham Palace said Prince Philip's admission to hospital was not related to coronavirus