The Duke of Edinburgh is expected to spend up to a fortnight in hospital after undergoing an abdominal operation yesterday in a development that has prompted fresh concerns for his health.
The Duke, who will turn 92 on Monday, attended the London Clinic by previous appointment for an exploratory operation to be carried out under general anaesthetic.
He was said to be in good spirits and walked into the hospital unaided after being driven there by car.
Earlier in the day he looked well as he attended a Buckingham Palace garden party and was also presented with the Order of New Zealand by the Queen.
Guests at the garden party said he had been telling jokes and showed no signs of being unwell.
Sources said the Duke was not suffering from any pain or discomfort and had arranged to have the operation after doctors "picked something up" during a routine health check in the past week.
It is understood that the Duke's condition is unrelated to the bladder infection he suffered during last year's Diamond Jubilee celebrations, which was thought to have been brought on by his prolonged exposure to the cold during the Thames River Pageant.
It was also said to be unrelated to the blocked coronary artery he suffered in 2011, when he spent Christmas in Papworth Hospital near Cambridge after being airlifted from Sandringham, where he had complained of chest pain.
A source said: "He has been carrying out engagements all week and his hospital trip was planned in advance.
"He was driven there by car, walked into the hospital and will have the operation tomorrow."
A two-week hospital stay would amount to the longest period the Duke has ever spent in hospital, but a Palace source said: "If anyone is going to have surgery at that age they are likely to be in hospital for that long."