Prince Philip has been rushed to hospital.
The 91-year-old royal - who has been staying at Balmoral in Scotland with his wife Queen Elizabeth - has been taken to Aberdeen Royal Infirmary as a "precautionary measure", according to Buckingham Palace.
No other details have been released.
This is the latest health scare for the Duke of Edinburgh, who was hospitalised in June with a bladder infection.
Philip was taken ill after spending hours on a ceremonial barge in rain and a cold wind as part of a flotilla of 1,000 vessels on the River Thames to mark Queen Elizabeth's landmark Diamond Jubilee.
Royal aides said he was treated with antibiotics and made good progress before being released six days later.
In December he had a heart operation where surgeons cleared a blocked coronary artery and inserted a stent to ease the problem.
Since then, Queen Elizabeth has tried to get him to slow down.
Queen Elizabeth's cousin Margaret Rhodes said: "It was horribly worrying for the queen when he had that little heart scare at Christmas. She was trying to make life easier for him, but he doesn't like that. He wants to be able to carry on in his old ways, which is wonderful, at his age. You go on doing something you've always done until for some reason you can't do it any more."
