The Duke of Edinburgh has missed the State Opening of Parliament and Royal Ascot after being admitted to hospital as a "precautionary measure" for treatment for an infection, Buckingham Palace said.
Philip was driven to a private hospital in London on Tuesday evening, following the advice of a doctor, after becoming ill with an infection arising from a pre-existing condition.
There is likely to be concern for the Duke given his age of 96, but he was out of his hospital bed earlier on Wednesday and a Palace spokesman said he is in "good spirits".
The 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 spokesman added: "Her Majesty is being kept informed and will attend Royal Ascot as planned this afternoon."
The Prince of Wales took the place of his father and accompanied the Queen to Westminster for the State Opening of Parliament.
Buckingham Palace would not go into details about Philip's condition or the type of infection, but his past history of a bladder infection could indicate his current problem.
The Duke was at the Queen's side on Tuesday for the opening day of Royal Ascot and a source said he was on "good form" during the event.
He and the Queen were joined by many members of their family, including Charles and Camilla, the Duke and Duchess of Cambridge, the Duke of York and the Earl and Countess of Wessex.
It is not known how long Philip will be kept in hospital but it could be a few days to allow any treatment to take effect.
He has enjoyed good health throughout much of his life, recovering from a blocked coronary artery in 2011 and a bladder infection in 2012, the latter leading him to miss the majority of the Queen's Diamond Jubilee celebrations and spend a period in hospital.
His decision to retire from public duties after the summer was not health-related, Buckingham Palace said at the time of the announcement earlier this year.
The Duke is an advocate of healthy eating combined with exercise. He once said he more or less followed the Atkins diet and he drinks only moderately.
He is well-known for his robust constitution. He told a flu expert at a research centre last year that he had not had the illness for 40 years.