The Duke of Edinburgh's youngest son said he was “getting better” after visiting him in hospital this afternoon.
Philip, 90, is suffering from a bladder infection and was taken to the King Edward VII Hospital from Windsor Castle yesterday as a precautionary measure.
The illness forced him to miss several events from the Queen's Diamond Jubilee celebrations, including a service of thanksgiving at St Paul's Cathedral today.
The Earl and Countess of Wessex arrived at the London hospital at 3.15pm, arriving in two separate vehicles, and stayed for just under half an hour.
The Countess of Wessex added: "He's in good spirits, he's on good form."
They took their children Louise and James with them.
A Buckingham Palace spokeswoman said the Duke will remain in hospital under observation for a few days.
He had been due to attend today's events at the Queen's side. The Queen instead travelled to St Paul's with one of her ladies-in-waiting, Diana Marion, the Lady Farnham, and sat next to the Prince of Wales during the service.
During the open top carriage ride back to Buckingham Palace this afternoon, she was accompanied by Charles and the Duchess of Cornwall.
The Prince of Wales paid tribute to his father last night as he addressed the crowds gathered for the Diamond Jubilee concert at Buckingham Palace.
"The only sad thing about this evening is that my father cannot be here with us because unfortunately he's been taken unwell," he said.
He added: "Ladies and gentlemen, if we shout loud enough he might just hear us in hospital."
A huge cheer went up from the spectators and many stamped their feet and chants of "Philip, Philip" broke out.
During Sunday's river pageant, the Duke spent several hours on the Thames and clearly enjoyed seeing the flotilla despite torrential rain.
Prime Minister David Cameron said the Queen and Duke would not have missed the river pageant despite any risk to their health.
Speaking to Sky News today, Mr Cameron said: "It was a most extraordinary spectacle and obviously everyone is very concerned and worried and wants to know that (the Duke) is going to be OK.
"The thing about our Royal Family is that they are incredibly dedicated to what they do and they are incredibly dedicated to doing it, no matter what the circumstances.
"I don't know all the conversations that took place but I would be pretty certain that Her Majesty and the Duke of Edinburgh would have wanted to do what they did and would have carried on doing it."
As he left the hospital, Edward told the huge pack of press outside: "He's getting better, he just needs some rest."
Asked if the Duke had been keeping up with today's Jubilee celebrations, he said: "He's been watching it on TV."
And asked how the Queen was doing without him, he said: "She's bearing up but she's missing him, obviously."
As he got into his Land Rover, he told reporters: "Thank you very much for your concern, it's much appreciated."