The Duke of Edinburgh was admitted to hospital on Tuesday as "a precautionary measure" for an infection arising from a pre-existing condition.
A file photo of Prince Philip, who was with the Queen at Royal Ascot on Tuesday.
Photo: AFP
Buckingham Palace said Prince Philip was being treated at the King Edward VII Hospital in London and was "up and about and in good spirits".
The duke, 96, said in May that he would retire from royal duties this autumn - a move supported by the Queen.
She is still carrying out her official engagements and on Wednesday delivered the Queen's Speech to Parliament before heading to the racing at Ascot.