Ed Miliband, who less than 24 hours before appeared to stand alongside the Prime Minister on the question of strikes against Syria, contacted Downing Street again to insist there had to be a "UN moment" - that no military action should take place before the UN inspectors reported back on their findings.
Westminster sources indicated his Labour colleagues believed he had previously given his support too cheaply and, with memories of Iraq, made it clear the UN route had to be followed to its natural conclusion.
There were suggestions party frontbenchers could have resigned, including Diane Abbott, the party's health spokeswoman.
Meantime, Ban Ki-moon, the UN Secretary General, urged the US not to rush to action and to "give peace a chance".
He said it would take another four days before the inspectors completed their work and reported back. This bolstered Labour in its argument to delay.
First Minister Alex Salmond also made clear his opposition to a swift military intervention, insisting the necessary conditions had not been met.
Tory backbenchers also expressed unease. David Davis, the former leadership contender, raised the prospect of defying the party whip. He warned there were huge risks to bombing Syria and that, as yet, he was not convinced the chemical attack was a deliberate act by the Assad regime.
As suggestions were made that a majority of Tory backbenchers might even rebel, Gerald Howarth, the former Conservative defence minister, warned of the risk that the UK and allies could "get our hand caught in the mangle" by intervening in Syria's civil war.
As doubts grew, a senior Coalition source admitted Mr Cameron could "pull the motion" if it looked as if the UK Government was going to face defeat.
Labour published its amendment not only calling for a delay until the UN inspectors reported back but also demanding the PM make public "compelling evidence" the Assad regime was responsible for the attack.
The Coalition then published its motion, deploring the chemical attack and urging a strong humanitarian response, which might require military action that was "legal, proportionate and focused on saving lives by preventing and deterring further use of Syria's chemical weapons".
However, it also noted how the UN process must be followed to give any action "maximum legitimacy", supported the inspectors reporting back to the Security Council and said every effort should be made to secure a Security Council Resolution backing military action before any such action was taken.
It added: "Before any direct British involvement in such action, a further vote of the House of Commons will take place."
Labour last night claimed Mr Cameron had been forced into to a climbdown by Mr Miliband.
A party source said: "At 5.15pm, David Cameron totally ruled out a second vote; an hour and a half later he changed his mind.
"Ed was determined to do the right thing. It has taken Labour forcing a vote to force the Government to do the right thing."
However, a Number 10 spokeswoman said the Prime Minister was "acutely aware of the deep concerns in the country caused by what happened over Iraq", that he respected the UN process and wanted to proceed "on a consensual basis".
His retreat means today's parliamentary debate is unlikely to be as heated as it could have been. Nonetheless, if the US simply delay any military strike to await Britain's decision, then the impassioned exchanges over any UK involvement will have simply been delayed for another day.