Theresa May and Donald Trump agreed the brutal use of chemical weapons in Syria must not go unchallenged in a fresh phone call between the two leaders on Thursday night. 

The high-level communication came after a two-hour meeting of senior ministers on the Prime Minister’s ‘War Cabinet’, in which she won their backing for action against Bashar Assad. 

May and the US President agreed a need “to deter the further use of chemical weapons by the Assad regime” and that they would work closely together on the international response.

Jeremy Corbyn has accused the government of ‘waiting for instructions’ Trump on what to do.

He 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’.”

Downing Street released a statement which said May and Trump had “agreed that the Assad regime had established a pattern of dangerous behaviour in relation to the use of chemical weapons”.

It went on: “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.”

Ministers had agreed it was “highly likely” Assad was responsible for the attack on Saturday on the rebel-held town of Douma – which reportedly left dozens dead, including children. Assad, as well as his allies in Iran and Russia, deny they were behind the strike. 

But the cabinet’s signal of support comes as May faces growing calls from opposition MPs to consult Parliament on backing US airstrikes. 

The White House, meanwhile, said Trump was also due to hold further talks with French President Emmanuel Macron. 

Earlier, Trump had appeared more cautious when backtracking on his explosive rhetoric on Syria to announce he “never said when” a strike on the country would take place.

He emerged from a meeting with his national security team without a “final decision” on how to respond. 

It is a far cry from his actions on Wednesday when his tweets raised international tensions, warning Russia that an attack in Syria was imminent. 

Just a day later, the US President said the attack could “be very soon or not so soon at all”, before asking why America had not received a “thank you” for ridding Syria of Islamic State (Isis). 

A Downing Street statement read:

“This afternoon Cabinet met and received an update on the attack against innocent civilians in Douma, Syria, on Saturday.

“The Prime Minister said it was a shocking and barbaric act which killed up to 75 people, including children, in the most appalling and inhumane way.

“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.

“The Prime Minister said it was a further example of the erosion of international law in relation to the use of chemical weapons, which was deeply concerning to us all.

“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.

“Cabinet agreed the Prime Minister should continue to work with allies in the United States and France to coordinate an international response.”

Against the threat of western forces from the UK, US and France coming into direct confrontation with Syria’s ally in Russia, Trump’s most recent statement contradicts a tweet sent on Tuesday in which he wrote: “get ready Russia”, warning that missiles “will be coming” to Syria.

Emmanuel Macron said France has proof the Syrian government carried out the attack, which aid groups have said killed dozens of people, and will decide whether to strike back when all the necessary information has been gathered.

German Chancellor Angela Merkel said the attack showed that Assad’s regime had failed to eradicate its chemical arsenal, but she ruled out joining any military action against Syria.

Russia’s military has denied that soil samples or other tests revealed the use of chemical weapons, as experts from the Organisation for the Prohibition of Chemical Weapons (OCPW) are due to arrive on Saturday.

The Kremlin said on Thursday that a crisis communications link – or “deconfliction” telephone line – with the US, was being used by both sides.

The Russian Foreign Ministry said Moscow sought no escalation of the situation, but that it could not support “dishonest accusations” and it had found no evidence of a chemical weapons attack in Douma.

The ministry urged the specialists to be deployed before a possible attack by US forces and its allies. 

“Any delay in visiting the location of the alleged incident could lead to another reckless move of Washington, which already fired missiles on Syria’s Shayrat airbase in April 2017 in violation of the UN charter and international law,” the ministry said in a statement.

On Wednesday Russia announced it would be sending ground troops to Douma and today it said control of the town had been transferred to Syrian government forces.