The Duke of Edinburgh was admitted to hospital on Tuesday as “a precautionary measure”, Buckingham Palace has said.
He is being treated at the King Edward VII Hospital, London, for an infection arising from a pre-existing condition.
The BBC understands his hospital stay is “not a cause for serious concern” and the palace said the duke 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.
It sets out the government’s legislative programme, this time covering a two-year period and dominated by laws about Brexit.
In a statement, Buckingham Palace said the Prince of Wales would instead accompany the Queen to Parliament and Prince Philip was “disappointed” to miss the event.
“Her Majesty is being kept informed …