Allied air strikes against the Syrian government over the alleged use of chemical weapons could be delayed until next week in the face of strong opposition in the UK parliament to British involvement in immediate military action.

The prime minister, David Cameron, conceded that MPs would be given a second vote to approve military action to defuse a parliamentary revolt, ahead of a Commons debate on Syria on Thursday. Whitehall sources indicated that the US, which had planned to launch the strikes by the weekend, is prepared to revive a back-up plan to delay the strikes until Tuesday when Barack Obama is due to set out for the G20 summit in Russia.

Such a move by the Obama administration would effectively hand Cameron a political lifeline after the opposition Labour party threatened to inflict a defeat on the Conservative-led coalition in parliament.

Obama, referring to the chemical attacks in a PBS television interview on Wednesday, said: "We have concluded that the Syrian government in fact carried these out."

The president said he had not taken a final decision on air strikes but Syria needed to understand there were "international consequences" for its actions. "If in fact we make a choice to have repercussions for the use of chemical weapons, then the Assad regime will have received a pretty strong signal that in fact it had better not do it again," Obama told PBS.

In an effort to build support for punitive strikes, the US and UK will on Thursday publish a joint summary of the intelligence which they say points towards the Assad regime's responsibility for the poison gas attack of 21 August in Ghouta, eastern Damascus, that killed over 1,000 people.

In a reflection of the different political pressures pulling the transatlantic allies in different directions, Downing Street undertook to return to the security council in a renewed effort to secure a UN mandate for military action after Russia blocked a British resolution at an informal meeting in New York. But the US state department meanwhile insisted it saw "no avenue forward" at the UN for finding an international consensus for armed action, because of Russian support for Bashar al-Assad's regime.

Furthermore, Washington made it clear it saw no need to wait for a report by UN inspectors currently in Damascus investigating the gas attack, estimated to have killed more than 1,000 people.

"We are going to make our own decisions on our own timelines about our response," the state department spokeswoman Marie Harf said. She added that because of initial Syrian government obstruction of the UN investigation, it had "passed the point where it can be credible".

However, the UK is now committed to wait for the UN report. The House of Commons will be asked by the government on Thursday to approve a "strong humanitarian response", possibly including force in principle. Direct action would depend on a second vote which in turn would be held after the UN weapons inspectors had reported back.

UN officials said the report could take another week or more to produce. The inspectors will continue to collect samples at the Ghouta site for the next four days, bringing their presence to the two weeks agreed with Damascus. The samples would then have to be subjected to laboratory analysis.

If the wait for the UN report extends much beyond Tuesday, the transatlantic ties could fray further, putting the prime minister under intense pressure. Cameron had faced the prospect of a defeat, or a politically damaging narrow victory, when MPs vote on Thursday evening on a motion calling for a proportionate response.

Syria warned of "grave consequences" if US-led military action goes ahead. Bashar al-Jaafari, Syria's ambassador to the UN, told reporters outside the security council in New York on Wednesday that the effect could be felt across the Middle East. "We should keep in mind what happened in Iraq and Libya", the envoy said, adding that the toppling of Libya's Muammar Gaddafi by Nato-backed rebels in 2011 had "spread terrorists all over Africa".

Jaafari urged the US, UK and France to back off and allow UN weapons inspectors to complete their investigation into last week's chemical attack outside Damascus. The sole purpose of the threat of airstrikes was "undermining the inspection team." Jaafari added: "We are not war mongers, we are a peaceful nation seeking stability in the area. The Syrian government is against the use of chemical weapons by all means – this is a moral obscenity."

Speaking in London the British foreign secretary, William Hague, said it was time for the UN to act. "This is the first use of chemical warfare in the 21st century. It has to be unacceptable, we have to confront something that is a war crime, something that is a crime against humanity. If we don't do so, then we will have to confront even bigger war crimes in the future."

The state department also gave more details of its intended justification for military action. A spokeswoman said Assad's alleged use of chemical weapons violated "the general law of war" while the use and proliferation of such weapons represented a threat to America's core national interests.

In his interview with PBS, Obama said that it was in America's "core self-interest" to prevent chemical weapons being used in a volatile area, near allies such as Turkey and Israel.

With as many as 70 Tory MPs threatening to rebel, British opposition leader Ed Miliband announced just after 5pm BST that he would instruct his MPs to vote against the government motion if a separate Labour amendment – calling for any action to be delayed – was defeated.

Within two hours the British government announced, as it published its motion for the debate, that a second vote would have to be held before Britain joins any military action. The motion says: "Before any direct British involvement in such action a further vote of the House of Commons will take place."

Downing Street was furious with Miliband and accused him of having suffered a giant "wobble" after he had appeared to indicate on Tuesday night that he would be prepared to support military action, subject to legal approval. But Labour hit back and said that the prime minister had been resisting a second vote until Miliband tweeted his plan to table his own amendment.

A Labour source said: "We will continue to scrutinise this motion but 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."

Downing Street said the prime minister offered a second vote because he wants to act in a consensual way. A spokesperson said: "The prime minister is acutely aware of the deep concerns in the country caused by what happened over Iraq. That's why we are committed to taking action to deal with this war crime – but taking action in the right way, proceeding on a consensual basis."

"So this motion endorses the government's consistent approach that we should take action in response to Assad's chemical weapons attack; reflects the need to proceed on a consensual basis, taking account of the work done by weapons inspectors; and reflects the prime minister's respect for the UN process – something he made clear to President Obama several days ago."

The No 10 move is likely to take the heat out of Thursday's parliamentary debate that will be opened by Cameron at 2.30pm and wound up by Nick Clegg, the deputy prime minister, at 10pm. The debate will be preceded by a meeting of the cabinet that will approve a recommendation from the National Security Council that Britain should join the military strikes. Dominic Grieve, the British attorney general, advised the NSC that such action would be legal under international law.

The National Security Council also agreed a specific plan for a British contribution to military action. This focused on a "limited one-off" operation and the measures that might have to be taken to protect British interests in the region, including the defence of the UK's sovereign base in Cyprus, which is thought to be potentially within range of President Assad's Scud missiles.

Though considered unlikely, sources said it was possible the US would act without British support – which would be a huge embarrassment for the prime minister. It would also be politically difficult for the White House. US officials have stressed that America would not act unilaterally, but in concert with partners.

France has pledged to take part in punitive action against the Assad regime, and its presidential system means that Francois Hollande, like Barack Obama is not obliged to consult the legislature.

However, British abstention would undermine Washington's claims of broad support.

Additional reporting by Ed Pilkington at the United Nations