Barack Obama also came under pressure to delay the widely-expected US intervention as Ban Ki-moon, the United Nations Secretary-General, pleaded for more time for its inspectors in Syria who are investigating last week's chemical weapons attack in the suburbs of Damascus, in which hundreds of civilians were killed.
"Let them conclude their work for four days and then we will have to analyse scientifically with experts. Then I think we will have to report to the Security Council for any action," he said.
Labour toughened its stance against UK military action only a day after Ed Miliband signalled that the Opposition was likely to support Mr Cameron. Tonight it demanded six concessions as the price of supporting him tomorrow night – including a UN Security Council vote on the inspectors' report; a further report to the Commons and a second vote before Britain takes part in any military action; "compelling evidence" that the Assad regime was responsible for last week's attack and a "clear basis in international law" for intervention.
Labour warned it would vote against the Government unless its conditions were met and Douglas Alexander, the shadow Foreign Secretary, said: "We are not prepared to issue a blank cheque to the Prime Minister."
The Opposition's change of tune could help delay the expected strikes against Syria, although President Obama could still decide to go ahead without British involvement. There had been speculation that UK forces could join an attack as early as this weekend but that now looks unlikely.
Labour claimed it had forced Mr Cameron to concede a second Commons vote before UK forces take part in any military intervention, which can now not take place until next week at the earliest. A Labour source said the Prime Minister had "totally ruled out a second vote" in a telephone call with Mr Miliband at 5.15pm tonight, only to agree to one when the Government's motion was published less than two hours later. The motion states the UN Security Council should have the opportunity to consider the inspectors' briefing and that "every effort" should be made to secure a UN resolution backing military action before it is taken.
Although Labour claimed Mr Cameron had backed down, it refused to withdraw its threat to oppose the Government tomorrow night. More than 80 Tory MPs have already signalled their anxiety about intervention in Syria.
However, ministers believe their concessions will reassure Conservative doubters and may yet win Labour's support. In another echo of the run-up to the Iraq War, the Government will today publish a statement by the Joint Intelligence Committee blaming the Syrian regime for the attack. Officials insisted it was an independent report, based on "open" rather than intelligence sources. They will not describe it as "dossier" because the Blair government was accused of putting pressure on the JIC to come up with evidence Saddam Hussein possessed weapons of mass destruction prior to the invasion of Iraq in 2003.
Before tomorrow night's vote, Mr Cameron will issue a summary of the Government's legal advice, which suggests military action could be taken without UN approval in order to protect the Syrian people. Dominic Grieve, the Attorney General, reported his view today to the National Security Council chaired by the Prime Minister, which declared Assad was responsible for the attack and "the world shouldn't stand idly by". In New York, Britain proposed a new UN resolution condemning last week's attack and authorising "measures to protect civilians" in Syria. But there was a stalemate when it was discussed with the other four permanent members of the Security Council, where Russia and China have a veto.
Marie Harf, deputy spokeswoman at the US State Department, conceded: "We see no avenue forward, given continued Russian opposition, to any meaningful council action on Syria. We do not believe that the Syrian regime should be able to hide the behind the fact that the Russians continue to block action." Sergei Lavrov, the Russian Foreign Minister, told Britain the Security Council should not consider a draft resolution before UN inspectors report on their findings.
William Hague, the Foreign Secretary, said there would be "further discussions in New York over the coming days" but added: "We are clear that if... there isn't agreement at the UN, then we still have a responsibility. It's very important not to take so long to respond that people confuse what the eventual response is about."
Elsewhere, when asked if Syria would strike Israel in retaliation at an attack on it, Bashar Jaafari, Syria's ambassador to the UN, said it had a "right to self defence under the UN charter", while Ayatollah Ali Khamenei, Iran's Supreme Leader, warned that US military action would be "a disaster for the region".
The Arab League blamed the chemical attack on Assad, but stopped short of condoning retaliation, while Jordan warned that it would not be used as a launching pad for any attacks on Syria. Nouri al-Maliki, the Prime Minister of Iraq, said the country was on high alert against any domestic consequences of possible military action in Syria.
