THERESA May has been told it would be “unforgivable” if she “ploughs blindly ahead” with taking the UK into the war in Syria.

The SNP’s Westminster leader Ian Blackford has demanded the Prime Minister recall Parliament tomorrow to allow MPs a vote on any change to the British military involvement in the country.

At a meeting in Downing Street last night Cabinet ministers backed May’s call to join military action threatened by the United States.

It is believed the Prime Minister is prepared to take action against the Assad regime without first seeking parliamentary consent.

May and her Cabinet met to discuss the British response to the alleged use of chemical weapons by the regime of Bashar al-Assad on the last rebel-held town of Douma, in Eastern Ghouta.

In a letter to May, Blackford warned the Prime Minister she had “no majority or mandate for military action”.

He told the Tory leader: “To plough blindly ahead with military intervention, without the backing of Parliament and, crucially, with no short or long-term plan; no specified, achievable outcomes; no clear, effective strategy that benefits civilians on the ground in Syria; and no indication of an exit strategy, would be wholly unforgivable.”

Parliament, he added, must have the “opportunity to debate and to vote on this most important matter”.

It’s not entirely clear that May has the numbers to push through the Commons any proposal to change the role of the British military in Syria, beyond their mandated targeting of Daesh.

There was support for the SNP’s position from Tory MP, and father of the House, Ken Clarke.

He told the BBC’s World at One:“In a modern parliamentary democracy you’ve got to have parliamentary approval if you have a planned policy decision to launch a military attack.”

He added: “The Government must get approval for such an important policy before it acts. As it happens they would get my support for a proportionate well-targeted attack that was hitting some significant facilities that were associated with the use of chemical weapons. If we don’t do that then Assad keeps trying his hand and he will feel uninhibited.”

At the same time as May was meeting with her Cabinet, there seemed to be something of a climb down from the White House.

On Wednesday Donald Trump had warned Russia and Syria to “get ready” for US missiles.

Twenty-four hours later, he tweeted: “Never said when an attack on Syria would take place. Could be very soon or not so soon at all!”

There were reports of officials in Washington and Moscow using a special “deconfliction” line to try to ease tensions.

The Kremlin’s spokesman, Dmitry Peskov, told reporters: “The line exists and it is active. In general, the line is used by both sides.”

Turkish president Tayyip Erdogan also acted as a go-between, talking to both Trump and Vladimir Putin.

Meanwhile, Labour leader Jeremy Corbyn told an election event in Derby that a political solution was needed.

“What happened in Syria is disgraceful. Any use of chemical weapons by anybody against anybody else is clearly illegal as well as immoral and wrong,” he said.

“The United Nations has a duty and a function to ensure there’s a proper investigation undertaken as the inspectors are now in Douma doing just that and, when we’ve got the results of that, decide what action to take.

“But, I would just say this. Hundreds of thousands have died and lost their lives in Syria.

“Millions have been forced into refuge. Many are living in terrible poverty and desperation. There has to be a political solution.

“Russia, America, the European Union, all the neighbouring countries, Iran, Saudi Arabia have got to be involved in ensuring there is a real ceasefire and a political process that does give hope to the people of Syria in the future.

“More bombing, more killing, more war will not save life. It will just take more lives and spawn the war elsewhere.”

Elsewhere, President Emmanuel Macron said France had proof that Assad was behind the chemical attacks.

He said: “We have proof that chemical weapons were used last week, at least chlorine, and that they were used by the regime of Bashar al-Assad”

“France will in no way allow an escalation or anything that would harm regional stability, but we cannot allow regimes that believe they can act with impunity to violate international law in the worst possible way.”

Assad, said Western threats to strike were based on “lies”.

Experts from the Organisation for the Prohibition of Chemical Weapons arrived in Syria last night with a view to starting investigations tomorrow.

“We will facilitate the arrival of the team to anywhere they want, in Douma, to check whether or not there was use of chemical substances,” Bashar Ja’afari, Syria’s ambassador to the UN , told reporters.

He denied there had been a chemical attack.