The House of Commons was suspended briefly today, and Prime Minister's Questions cancelled, as a "mark of respect" following the death of Tory leader David Cameron's disabled son Ivan.
Prime Minister Gordon Brown was joined at noon by shadow foreign secretary William Hague and Liberal Democrat deputy leader Vince Cable in delivering sombre messages of sympathy and condolence to Mr Cameron and his wife Samantha, in place of the usual exchanges.
Speaker Michael Martin then adjourned the House until 12.30pm.
Ivan, six, who suffered from cerebral palsy and a rare and severe epilepsy syndrome, was taken ill overnight and died at St Mary's Hospital in Paddington, west London.
Mr and Mrs Cameron have two other children, Nancy, five, and Arthur, three.
Mr Brown, whose own baby daughter Jennifer Jane died aged just 10 days old in 2002, expressed his and his wife Sarah's sorrow at Ivan's death.
The premier, wearing a black tie, told MPs: "I know that the whole House will want to express our sorrow at the sad death this morning of Ivan Cameron at the age of just six years old.
"I know that in an all too brief life, he brought joy to all those around him and I know also that for all the days of his life, he was surrounded by his family's love.
"Every child is precious and irreplaceable and the death of a child is an unbearable sorrow that no parent should ever have to endure.
"Politics can sometimes divide us. But there is a common human bond that unites us in sympathy and compassion at times of trial and in support for each other at times of grief.
"Sarah and I have sent our condolences to David and Samantha and I know the whole country - our thoughts and our prayers - are with David, Samantha and their family today."
Mr Hague, who spoke to Mr 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 added: "Ivan their son suffered much in his short life but he brought joy and love to those around him.
"As David himself has said in the past, for him and Samantha he will always be their beautiful boy."
Mr Hague said 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".
Mr Cable added: "I think everybody in the House has experienced bereavement, but there is something especially sad and shocking about the loss of a child.
"We recognise, I think all of us, that this is something that is especially difficult to cope with."
Speaker Martin then told MPs: "This House will share with me its sadness at this news and our hearts and sympathies go out to David and Samantha and to Nancy and Arthur.
"As a mark of respect to Ivan this House will suspend until 12.30pm this day."
The suspension of PMQs and normal Commons business usually only follows the death of a party leader or former premier.
But shortly after learning of the Camerons' loss, Mr Brown offered to cancel today's question time session.
The last time Prime Minister's Questions was suspended because of a bereavement was on 12 May 1994, following the unexpected death of Labour leader John Smith, aged 55, from a heart attack earlier that day.
Downing Street also announced that the formal unveiling of a new portrait of former prime minister Lady Thatcher, which Mr Cameron had been due to attend at Number 10 later, had been postponed.
Asked about the suspension of question time, Mr 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."
A Conservative Party spokesman said: "Number 10 contacted David Cameron's office and offered to suspend PMQs. We at the time were preparing for PMQs with William Hague but we gratefully accepted Gordon Brown's offer."