The sudden death of Tory leader David Cameron's son Ivan, who had been taken to hospital in London, was "very, very sad and shocking", said shadow chancellor George Osborne.
The six-year-old, who suffered from cerebral palsy and epilepsy and required round-the-clock care throughout his life, died early on Wednesday morning at St Mary's Hospital in Paddington, London.
Mr Osborne, a close friend of the Camerons, said Ivan's disability "does not make today's very, very sad and sudden death any less shocking".
"Sadly he has been a very ill boy since the moment he was born ... but nevertheless he was always at the centre of family life round at the Camerons," he told Sky News
"He quite regularly had seizures and problems and would have to go to hospital. He had such a seizure this morning and very sadly died shortly after admission to hospital.
"It has left the Cameron family in a great deal of grief and shock."
Messages of sympathy flooded in following the announcement of Ivan's death.
The Queen sent a private message of sympathy to the Camerons, Buckingham Palace said.
Prime Minister Gordon Brown and his wife Sarah also sent a message of sympathy to Mr Cameron and his wife.
The Prime Minister said: “Sarah and I were very saddened to hear of the death of Ivan and we have sent our condolences to David and Samantha. The death of a child is a loss no parent should have to bear.”
Mr Brown’s own daughter Jennifer Jane died when just a few days old in 2002.
He added: “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.”
A Conservative spokesman said it was "probably the most difficult day of his (Mr Cameron's) life".
He said: "It is with great sadness that David and Samantha Cameron must confirm the death of their six-year old son Ivan. 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.”
As a mark of respect Prime Minister’s Questions, due at midday every Wednesday when the Commons is sitting, was cancelled. Instead, Mr Brown, William Hague, Mr Cameron deputy, and Vince Cable for the Liberal Democrats, made short statements before the sitting was suspended until 12.30pm when normal business resumed.
Mr Brown told a subdued Commons that the whole House would want to express its sorrow at the "sad death" of Ivan Cameron.
He said every child was "precious and irreplaceable" and that the death of a child was something that no parent should have to bear.
Mr Hague thanked the Prime Minister on behalf of the Cameron family for his words, and for suggesting the suspension of Prime Minister's Question Time.
Mr Hague said the Prime Minister would understand the "dimensions of the loss" as much as anyone in the House, and also expressed Mr Cameron's gratitude to members for their sympathies.
He added that Ivan's six years of life "were not easy", and the family had known he could die young. Mr Cameron and his wife also wanted to convey their gratitude to the NHS workers who had "done their best" for the boy throughout.
Mr Cameron’s constituency agent, Barry Norton, said: “Ivan’s sudden passing has come as a huge shock. David and Samantha will be devastated. They were deeply devoted to this lovely boy, who brought so much joy to their lives.”
Mr Norton, who is leader of West Oxfordshire District Council, added: “All our thoughts and heartfelt sympathies are with them and their children Nancy and [Arthur] Elwen at this heartbreaking time for them.”
Boris Johnson, the Conservative London Mayor Boris Johnson said, “I was shocked at the terrible News
this morning of the death of Ivan Cameron.
”My deepest sympathies are with David, Samantha, Nancy and Arthur.”
Ivan’s condition means that it was necessary for his parents and his carer to take it in turns to be with him throughout the night.
Mr Cameron is also said to have spent many nights lying by his son’s bedside in hospital before going in to work the next morning. He occasionally had to take naps during the day to catch up on sleep.
He has described his family as the most important facet of his life, placing it clearly ahead of his ambition to run the country.
”The fact is that whatever I do or don’t achieve in politics, nothing matters as much as my family. Nothing is tougher, or will give me greater satisfaction, than raising my children properly,” he wrote in a newspaper article a few months after becoming Tory leader.
”And it goes without saying that my children couldn’t care less whether I become prime minister or not,” he added. THE TELEGRAPH