Conservative leader David Cameron's eldest son Ivan has died in hospital.
The six-year-old, who had cerebral palsy and epilepsy, became ill overnight and died at St Mary's Hospital in Paddington, London.
Mr Cameron, who described Ivan as "wonderful", and wife Samantha have two younger children, Nancy and Arthur.
Prime Ministers' Questions, which Mr Cameron had been due to attend at noon, was called off at Prime Minister Gordon Brown's suggestion. In place of the normal clashes between MPs and the prime minister, Mr Brown expressed his condolences to the Camerons on behalf of the whole House and country.
Shadow Foreign Secretary William Hague, standing in for Mr Cameron, thanked Mr Brown "on behalf of David and his family" for his "heartfelt words".
Deputy Lib Dem leader Vince Cable, who is standing in for Nick Clegg, also expressed his condolences in a short statement.
Commons Speaker Michael Martin then suspended the sitting until 1230 GMT "as a mark of respect to Ivan".
Mr Cameron had also been expected to join Mr Brown and Baroness Thatcher at 10 Downing Street later for the unveiling of a new portrait of her, but this event has also been called off.
Announcing Ivan's death earlier, a Conservative Party statement 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."
Prime Minister Gordon Brown said he and his wife Sarah were "very saddened" about Ivan's death.
In a statement, Mr Brown said: "We have sent our condolences to David and Samantha. The death of a child is a loss no parent should have to bear.
David Cameron was with Ivan when he met the Archbishop of York on Christmas day
"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."
Mr Brown and wife Sarah lost a daughter, Jennifer Jane, in 2002, who was 10 days old when she died in hospital in Edinburgh.
Mr Cameron, who has been leader of the UK's official opposition since 2005, had been an MP for Witney, in Oxfordshire, for less than a year when Ivan was born in April 2002.
He suffered from Ohtahara syndrome, a very rare form of epilepsy characterised by spasms which start in the first days of life. Some children can suffer as many as 100 seizures everyday.
Describing the moment when he learned of Ivan's disabilities, Mr Cameron told the Sunday Times in 2005: "The news hits you like a freight train.
"You are depressed for a while because you are grieving for the difference between your hopes and the reality. But then you get over that, because he's wonderful."
The BBC's political editor Nick Robinson said Mr Cameron's experience of looking after Ivan, who needed round-the-clock supervision, had shaped Mr Cameron's character and his attitude to the NHS.
Friends have previously attributed Mr Cameron's "modernising" leadership of the Conservatives to the NHS care Ivan received.
Ivan Cameron with Samantha Cameron
The illness has also helped the Tory leader realise that there is more to life than politics, they have said.
In a 2007 speech, Mr Cameron said: "I help care for a severely disabled child - my son.
"It's what I do at the start of each day. It's sharpened my focus on the world of care assessments, eligibility criteria, disability living allowance, respite breaks, OTs, SENCOs, and other sets of initials.
"But I would not dare to call myself a carer. The work that full-time carers or those with little extra help do is unbelievable."
London Mayor Boris Johnson, an old school friend of Mr Cameron, 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."
Senior Conservatives in Oxfordshire, where Mr Cameron's Witney constituency is located, also expressed deep sympathy for the Camerons.
Keith Mitchell, leader of Oxfordshire County Council, said: "David and Samantha have been inspirational in their care and support for Ivan. This will be a very sad blow for them both." Shadow Foreign Secretary William Hague will stand in for Mr Cameron as Conservative leader while he takes time off, it is understood.