David Cameron and his wife, Samantha, thanked well-wishers today for their messages of support following the death of their son, Ivan.

The six-year-old, who was severely disabled, was taken ill overnight and died at St Mary's Hospital, Paddington, early this morning.

A statement posted on the Conservative party's website said: "David and Samantha are grateful for all the warm messages of support and condolence that they have received."

The couple asked anyone considering sending flowers to instead make a donation to one of five charities: Mencap, Friends of St Mary's Hospital, Paddington, Friends of Jack Tizard School, Helen & Douglas House, Twickenham, and Shooting Star children's hospice.

Prime minister's question time was cancelled this afternoon, and Brown and other senior politicians instead offered their condolences in the Commons chamber.

"I know that the whole house will want to express their sorrow at the death of Ivan Cameron," Brown told MPs.

"He brought joy to all those around him. Every child is precious and irreplaceable. The death of a child is an unbearable sorrow no parents should have to endure."

The Browns' first child, Jennifer Jane, was born prematurely in late 2001 and died after 10 days. They have since had two sons, Fraser, who has cystic fibrosis, and John.

For the Tories, William Hague, who spoke to Cameron before making his statement, said: "Ivan's six years of life were not easy ones.

"His parents lived with the knowledge for a long time that he could die young but this has made their loss no less heartbreaking … He will always be their beautiful boy."

The Camerons were "hugely grateful" to the NHS care workers who "not only did their utmost for their son this morning but have helped every day since he was born", he said.

The birth of Ivan in 2002 had a profound effect on Cameron. The Tory leader has admitted that learning about his son's illness hit him "like a freight train", but his friends believe the experience of caring for Ivan gave him a much deeper understanding of the difficulties faced by people with a less sheltered upbringing.

Ivan needed care at night, and Cameron and his wife often took turns sleeping in his room.

He also needed constant visits to hospital, and Cameron's passionate commitment to the NHS – which has marked him out from other Tory leaders – was largely influenced by the contact he has had with his son's carers.

The Camerons had to adapt their west London home to accommodate Ivan, who needed a lift to get in and out of his bath.

Like Tony Blair, but unlike Brown, Cameron has always been keen to talk about his children and to be photographed with them.

Ivan was not able to talk, but Cameron spoke with obvious delight about the pleasure he got from seeing his son smile at him.

The Camerons also have a daughter, Nancy, five, and another son, three-year-old Arthur.

Although the Camerons have had "scares" in the past when Ivan's condition has been particularly bad – at one stage last year Cameron had to cancel a party visit because Ivan was in hospital – the family had had no warning that his life was in danger until he was suddenly taken ill.

It is understood Cameron and his wife were both with Ivan when he died.

A party spokesman said: "They had no particular warning that this tragedy was going to happen. Ivan had a bad night, but he's had bad nights before.

"They got him to hospital. But he did not pull through."

The spokesman said the mood in Conservative headquarters was very sombre.

"People are very, very upset," he added. "They knew that David cared for Ivan very, very deeply. David brought him to Westminster occasionally and lots of his staff had met him."

The Tory leader has postponed a planned appearance with Lady Thatcher at 10 Downing Street this evening for the unveiling of a new portrait of her, and party officials said he might cancel public engagements for the next two weeks.

The Liberal Democrat leader, Nick Clegg, whose wife gave birth to their third son at the weekend, said: "My heart goes out to David and Samantha at this incredibly difficult time for them and their family."

Asked about the suspension of today's question time, Brown's spokesman said: "The prime minister's view is that he wanted to consult the opposition parties and listen to their views and agree to a sensible way forward which marks the very difficult day it is going to be for everybody at Westminster."

Prime minister's questions was last cancelled was in 2000, when debate on a controversial bill ran overnight into the morning, and before that in 1994, on the day John Smith, the then Labour leader, died.