The severely disabled six-year-old son of British opposition leader David Cameron has died, a Conservative party spokesman said today.

"It is with great sadness that David and Samantha Cameron must confirm the death of their six-year-old son Ivan," a statement said.

"Ivan, who suffered from cerebral palsy and severe epilepsy, was taken ill overnight and died at St Mary's Hospital, Paddington, early this morning.

"David and Samantha would ask that their privacy is respected at this terribly difficult time."

The boy could not walk or talk and had to be fed via a tube through his stomach.

Cameron, who opinion polls tip to be the next prime minister of Britain, spoke freely of his son's illness in interviews and was pictured with Ivan and the rest of his family on his official Christmas card last year.

Asked by the Daily Telegraph newspaper last year if he was worried that his son might suddenly die, Cameron said: "You're quite philosophical about it.

"You know that children like Ivan never make it to old age. You live by the day. But he doesn't want to give up. This little person just wants to keep going."

Ivan was Cameron's eldest child

Prime Minister Gordon Brown said he was "very saddened" to hear that the boy had died.

"The death of a child is a loss no parent should have to bear," he said in a statement released by his Downing Street office.

"I know Ivan was a child who brought joy to all those who knew him and his was a life surrounded by love. The thoughts and prayers of the whole country are with David, Samantha and their family."

Brown's first child Jennifer died aged just 10 days in 2002 after being born prematurely and suffering a brain haemorrhage. His second son Fraser, two, has been diagnosed with cystic fibrosis.

Opinion polls place the Conservatives well ahead of Brown's ruling Labour Party. The next general election must be held by the middle of next year.

Cameron and his wife Samantha, creative director for luxury goods shop Smythson, have two other young children - Nancy, five, and Elwen, three. Ivan was their eldest child.