The Duke of Edinburgh is to spend a second night in hospital being treated for an infection which forced him to miss the State Opening of Parliament and Royal Ascot.
Philip was admitted to a private hospital in London on Tuesday evening as a precautionary measure, 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 was in "good spirits''.
The Prince of Wales said his father was "getting better" following a meeting with faith leaders at a Finsbury Park mosque in north London on Wednesday afternoon.
Charles took the place of his father and accompanied the Queen to Westminster for the State Opening of Parliament on Wednesday.
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.
A Buckingham Palace spokeswoman said on Wednesday evening: "The Duke of Edinburgh continues to be in good spirits and will stay in hospital overnight."
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.
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.
In a statement issued on Wednesday morning Buckingham Palace said: "The Duke of Edinburgh was admitted to King Edward VII Hospital in London, as a precautionary measure, for treatment of an infection arising from a pre-existing condition."