The Duke of Edinburgh was admitted to hospital last night 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 is “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 on Tuesday morning, Buckingham Palace said the Prince of Wales would instead accompany the Queen to Parliament and Prince Philip was “disappointed” to miss it.
“Her Majesty is being kept informed and will attend Royal Ascot as planned” on Wednesday afternoon, the palace said.
The duke was at the Queen’s side on Tuesday afternoon for the opening day of the four-day race meeting.
It is understood he was not admitted to hospital as an emergency but, on a doctor’s advice, was driven from Windsor Castle to the central London hospital on Tuesday evening.
How long he will remain there is not yet known.
Prince Philip has spent much of his life in good health – being treated for a blocked coronary artery in 2011 and a bladder infection in 2012. That saw him miss much of the Queen’s Diamond Jubilee celebrations.
He backs healthy eating and exercise. He drinks moderately, takes the stairs and says he still fits into the uniform he wore on the day he married the Queen, almost 70 years ago.