Mr Obama made clear that he had the authority to order a strike in response to the Assad regime using chemical weapons against civilians, and stands ready to do so at any time. But announcing a delay that stunned many, he pledged: "I will seek authorisation for the use of force from the American people's representatives in Congress. We should have this debate because the issues are too big for business as usual."

In a statement from the Rose Garden that was watched by the world, Mr Obama reiterated the US intelligence conclusion that the Syrian regime was responsible for the gas attacks. "This attack is an assault on human dignity," he declared, and warned that it could lead to escalating use of chemical weapons.

"This menace must be confronted. But having made my decision as commander-in-chief, I am also mindful that I am president of the world's oldest constitutional democracy."

With Congress not due to return to Washington until 9 September, the decision means any attack will not happen before the G20 summit in St Petersburg later this week. His about-turn will sit well with many members of Congress, with the UN, and even, to a degree, with Russia, a Syrian ally.

Polls in the US suggest the public is divided on the wisdom of strikes against Syria and, by a very wide margin, convinced that Mr Obama should at least consult Congress first. The decision is nonetheless a high-stakes gamble by the President. While he may win Senate support, the House of Representatives may baulk. Thus he could be stymied by Congress as David Cameron was by Parliament. There is also the risk that the Assad regime will use the pause to commit new atrocities.

But he may have bought himself some goodwill on Capitol Hill. "The President's role as commander-in- chief is always strengthened when he enjoys the expressed support of the Congress," noted Senator Mitch McConnell, the Republican minority leader in the Senate. In a statement the leadership of the majority Republicans in the House applauded the decision, but said debate would address "serious, substantive questions being raised".

In the speech, the President described Britain as "our closest ally" and acknowledged that what had happened in Parliament last week had shaped his strategy. In a tweet last night, Mr Cameron stated: "I understand and support Barack Obama's position."

The announcement will be viewed in No 10 as a victory for Mr Cameron's approach, despite the mistakes about parliamentary tactics and the misreading of the Conservative Party's mood. However, Congress will be emboldened by how Parliament voted last Thursday. There will also be relief in No 10 and the Foreign Office at the President's reaffirmation of the special relationship, after the US Secretary of State, John Kerry, earlier rebuffed Britain and described France, willing participants in a strike, as "our oldest ally".

Mr Obama's move means the issue will likely dominate the G20 summit, distracting from other issues, including Egypt. It will give Vladimir Putin, the Russian President and summit host, a stage on which to counter Mr Obama. Speaking earlier yesterday, he bluntly challenged the logic of the US intelligence. "While the Syrian army is on the offensive, saying that it is the Syrian government that used chemical weapons is utter nonsense," he said. "I would like to address Obama as a Nobel Peace Prize laureate: before using force in Syria, it would be good to think about future casualties."

On Friday, the US administration released an unclassified version of its intelligence assessment, concluding it had "high confidence" that the regime was responsible for the gas attacks. Mr Kerry gave a passionate defence of the administration, saying 1,429 people had been killed, far higher than earlier estimates.

The legal justification of executing strikes without congressional authority (or UN mandate) was already fuzzy. But by deferring to Congress, Mr Obama is setting a precedent that he – and future presidents – may come to rue, particularly if it is interpreted as a surrender of executive authority.

Confirming that a team of UN inspectors had pulled out from Syria earlier in the day, a UN spokesman rejected as "grotesque" any notion it had done so to give space for US strikes. The team crossed into Lebanon by car and flew to the Netherlands with samples gathered in the affected areas of the Damascus suburbs. The material includes urine and blood samples from victims, and soil samples.

The UN had urged Mr Obama to await the inspectors' findings. "The UN mission is uniquely capable of establishing in an impartial and reliable manner the facts of any use of chemical weapons based directly on evidence collected on the ground," the spokesman, Martin Nesirky, said, adding that results will provide "a picture of what happened". But officials with the Organisation for the Prohibition of Chemical Weapons in the Hague said it could take three weeks to compile final results.

For the Syrian opposition, the pull-back by the US will be a huge disappointment. Only hours earlier Qassim Saadeddine, spokesman for the rebels' Supreme Military Council, spoke to Reuters of their readiness to take advantage of any injury to the regime. "The hope is to take advantage when some areas are weakened by any strikes. We ordered some groups in each province to ready their fighters for when the strike happens."

It also represents a new challenge for President François Hollande whose commitment to support America may now face a blowback in a parliamentary debate later this week. A survey released by Le Parisien showed 64 per cent of respondents opposed military action, 58 per cent did not trust Mr Hollande to conduct it, and 35 per cent feared it could "set the entire region ablaze".

Following the announcement that the US president would seek a vote from congress, the British Prime Minister took to Twitter to endorse Mr Obama's approach, writing: "I understand and support Barack Obama's position on Syria."

Foreign Secretary William Hague followed, tweeting: "A fine speech by the President of the United States on Syria".

Former Liberal Democrat leader Lord Ashdown raised the prospect of British support being put back on the table, telling the BBC parliament could "reconsider its position".

"This was a brave and principled act from a brave and principled president," he added.