The Prime Minister is promising a further parliamentary vote before "any direct British involvement" in military action.
As a result, if the US Government goes ahead this weekend with military strikes on Syria then UK forces will not be part of them.
Westminster sources suggested political pressure on Labour leader Ed Miliband from party colleagues to qualify his support for possible military action, coupled with rumbles of rebellion among Tory backbenchers, led Mr Cameron to realise he could face a humiliating defeat.
A Labour source declared a political victory, saying: "Ed was determined to do the right thing. It has taken Labour forcing a vote to force the Government to do the right thing."
Number 10 insisted Mr Cameron, who had earlier chaired a meeting of the National Security Council, wanted to move on a "consensual basis" and the promise of a second vote reflected his "respect for the UN process".
Among those opposing swift military action was Alex Salmond.
The First Minister insisted that the necessary criteria of clear evidence within a legal framework and full consideration of the aims, objectives and consequences of any action had "not been met and therefore the case for military action in Syria - or the UK's participation in it - has not yet been made".
Earlier, the UK Government tabled a fresh UN resolution, saying the body should consider "what is necessary" to deter another chemical attack.
Expecting a Russian veto, William Hague, the Foreign Secretary, made it clear that Western allies could bypass the Security Council, saying: "We and other nations still have a responsibility."
Wael al Halqi, Syria's Prime Minister, warned that his country may become a "graveyard of invaders".