"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."

Question time was cancelled as a mark of respect.

Ivan could not walk and had to be fed via a tube through his stomach.

Mr Cameron, whom opinion polls tip to be the next prime minister, 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 last year if he was worried that his son might suddenly die, Mr 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."

The Camerons have two other young children

Opinion polls put the Conservatives well ahead of the ruling Labour Party led by the Prime Minister, Gordon Brown.

Mr Brown's first daughter Jennifer died aged just 10 days in 2002 after being born prematurely.

Agence France-Presse