David Cameron has opened his heart publicly for the first time over the sudden death of his son - telling well-wishers the loss of Ivan had left a hole in family life "so big that words can't describe it".

The six-year-old, who suffered from a rare epilepsy condition accompanied by severe cerebral palsy, died in hospital on Wednesday morning after being taken ill overnight.

The Witney MP and leader of the Conservative Party, who has two other children with wife Sam, has kept out of the public eye since the loss but used a weekly email to party activists, members and staff to thank them for their support.

In an emotional message, he said the couple had always been acutely aware that Ivan might die young.

"But we didn't expect to lose him so young and so suddenly. He leaves a hole in our life so big that words can't describe it.

"Bed time, bath time, meal time - nothing will feel the same again," he wrote. "We console ourselves knowing that he won't suffer anymore, that his end was quick, and that he is in a better place. But we all just miss him so desperately."

The Opposition leader said the family had been lucky to have enjoyed the time they had with "such a wonderfully special and beautiful boy".

"When we were first told the extent of Ivan's disability I thought that we would suffer having to care for him, but at least he would benefit from our care.

"Now as I look back I see that it was all the other way round. It was only him that ever really suffered and it was us - Sam, me, Nancy and Elwen - who gained more than I ever believed possible from having and loving such a wonderfully special and beautiful boy."

Expressing his gratitude for the support the couple have received, he wrote: "Sam and I have been overwhelmed by all the letters, cards, emails and flowers we have received about Ivan.

"Sending an e-mail this week just gives us both a chance to say a big 'thank you'. It means a lot to know that others are thinking of us and him."

Ivan was the couple's eldest child and had required round-the-clock care all his life. Mr and Mrs Cameron have two other children, Nancy, five, and Arthur Elwen, three.