Prince Philip was taken to hospital on Tuesday night as a "precautionary measure".
It related to an infection from a pre-existing condition, Buckingham Palace said.
The news came shortly before the Queen's Speech at the state opening of Parliament on Wednesday.
Philip was supposed to be accompanying the Queen to the State Opening of parliament on Wednesday but his place has been taken by the Prince of Wales.
A Buckingham Palace spokesman said: "The Duke of Edinburgh was admitted to King Edward VII Hospital in London last night, as a precautionary measure, for treatment of an infection arising from a pre-existing condition.
"Prince Philip is in good spirits and is disappointed to be missing the state opening of Parliament and Royal Ascot.
"The Prince of Wales will accompany the Queen to the state opening.
"Her Majesty is being kept informed and will attend Royal Ascot as planned this afternoon."
In May it was announced the Duke of Edinburgh would step down from public engagements in the autumn.
"The Duke will not be accepting new invitations for visits and engagements, although he may still choose to attend certain public events from time to time," the Palace said at the time.
Additional reporting by agencies
