The Duke of Edinburgh has never been far from the Queen's side, but over the course of the last two years the 91-year-old has frequently missed engagements due to illness and been admitted to hospital.
In December 2011 Prince Philip was treated at Papworth Hospital, near Cambridge, after undergoing heart surgery to have a stent fitted.
He was discharged after five days to rejoin the Queen and other members of the royal family at Sandringham.
The Duke was admitted to the King Edward VII Hospital in central London for several days after a bladder infection after the Diamond Jubilee River Pageant which culminated in an extravaganza on 5th June 2012.
However he was released in time to celebrate his 91st birthday with the Queen.
The Duke missed the Opening Ceremony of the Paralympic Games on 29th August 2012, after being admitted to hospital for a recurrent bladder infection.
Prince Philip remained at Balmoral to recover.
Buckingham Palace had played down fears about the Duke's health after he was pictured with a badly bruised eye during his visit to Canada.
He was understood to have woken up with the bruising under his right eye a few days ago. A palace spokeswoman said at the time that the Duke was "well".