Theresa May and Donald Trump have agreed that the use of chemical weapons must not go unchallenged after the Prime Minister won the backing of her Cabinet for action to prevent their further use in Syria.
The two leaders also said there was a need “to deter the further use of chemical weapons by the Assad regime”, as they pledged to work together on the international response to the suspected chemical weapons attack.
Meanwhile, Labour leader Jeremy Corbyn has accused the Government of “waiting for instructions” from the US and that military intervention risks “escalating an already devastating conflict”.
Ministers have said it is “highly likely” Syrian President Bashar Assad was responsible for the attack on Saturday on the rebel-held town of Douma which reportedly left dozens dead and that there was agreement around the Cabinet table that such actions should not go “unchallenged”.
Mrs May and President Trump spoke hours after this meeting and reiterated the use of chemical weapons should not be tolerated.
The largest US air and naval strike force since the 2003 Iraq war was said to be heading towards Syria, according to reports in The Times, paving the way for strikes within the next three days.
Russia has been granted a request for the United Nations Security Council to meet on Friday for fresh discussions on the threat to international peace from air strikes on Syria.
The statement released by Downing Street after Thursday’s Cabinet meeting made no direct reference to military action, but will be seen as a signal Britain would be prepared to join any US-led air strikes against the regime should the Americans decide to go ahead - putting it on a potential collision course with Assad’s principal backer, Russia.
In a statement released on Thursday night, a Downing Street spokeswoman said: “The Prime Minister spoke to President Trump about Syria this evening.
“They agreed that the Assad regime had established a pattern of dangerous behaviour in relation to the use of chemical weapons.
“They agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime.
“They agreed to keep working closely together on the international response.”
But Mr Corbyn said: “Further UK military intervention in Syria’s appalling multi-sided war risks escalating an already devastating conflict.
“The Government appears to be waiting for instructions from President Donald Trump on how to proceed. But the US administration is giving alarmingly contradictory signals.
“Even US defence secretary James Mattis has said we ‘don’t have evidence’ and warned further military action could ‘escalate out of control’.”
Earlier, President Trump appeared to row back from a suggestion on Wednesday that missile strikes were imminent, insisting in his latest tweet that he had never set out a timetable for military action.
“Never said when an attack on Syria would take place. Could be very soon or not so soon at all!” he wrote.
US broadcaster NBC quoted US officials familiar with the intelligence as saying they had now obtained blood and urine samples which had tested positive for chemical weapons.
That assessment appeared to echo President Macron, who said they had “proof” that “at least chlorine” was used in the attack by the regime.
The White House said it was still assessing the evidence after its security council meeting on Thursday.
The No 10 statement issued following the Cabinet meeting said Mrs May had again described the attack on Douma as a “shocking and barbaric act” which represented a further erosion of international law.
“Cabinet agreed that the Assad regime has a track record of the use of chemical weapons and it is highly likely that the regime is responsible for Saturday’s attack,” it said.
“Following a discussion in which every member present made a contribution, Cabinet agreed it was vital that the use of chemical weapons did not go unchallenged.
“Cabinet agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime.”
The statement made no reference to whether Parliament would be given a say on military action - prompting renewed concerns among opposition parties and some Tory MPs that Mrs May is prepared to go ahead without a Commons vote.
Mr Corbyn insisted that MPs were entitled to a vote, saying Parliament “must be consulted”.
Ken Clarke, the Conservative former chancellor, Liberal Democrat leader Sir Vince Cable and Ian Blackford, leader of the SNP in Westminster, also urged Mrs May to give MPs a vote on military action.
A team from the Organisation for the Prohibition of Chemical Weapons is due to start its investigation in Syria on Saturday.
On Friday campaigners from the Stop the War Coalition will hand in a letter signed by MPs, trade unionists, celebrities and academics to Downing Street urging Mrs May to not take military action in Syria.