Downing Street said an “international response” would be coordinated with allies France and the US, to show that the use of toxic weapons in the Middle Eastern country’s civil war would not be tolerated.
The cabinet agreed that Syrian leader Bashar al-Assad had a track record of using chemical weapons and was “highly likely” to have been behind last Saturday’s suspected chemical weapons attack in the town of Douma.
It comes as the armed forces of the Western allies, Syria and Russia all prepared for action, with The Independent understanding that missiles could be fired imminently.
During the two-hour cabinet meeting, in which every member of Ms May’s top team is said to have spoken, the prime minister said the Douma incident was a “shocking and barbaric act” which killed dozens of innocent people.
A readout from the urgent gathering called by the prime minister amid rising tensions said Ms May had explained that the attack was “a further example of the erosion of international law in relation to the use of chemical weapons”.
The readout went on: “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.”
Ms May later spoke to US President Donald Trump. A Downing Street spokesperson said 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."
Some military forces from Britain, France and the US are already in place to strike Syrian regime targets, with others reported to be manoeuvring into position.
Syrian government forces are said to have been moving out of harms way and preparing defences, while Russia has said it will take action to protect its personnel in Syria, raising the prospect of a clash between Russian and Western military assets.
The Downing Street readout said: “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.”
It is possible that an attack could come as early as Thursday night, with British attack submarines said to be on standby in the Mediterranean Sea, while RAF fighter jets based in Cyprus could also be used.
But an early attack would intensify domestic pressure on the prime minister, who faces demands from her own back benches and from Labour’s Jeremy Corbyn and other party leaders to allow parliament a vote prior to any action.
Former Tory chancellor Ken Clarke said earlier on Thursday: “In a modern, parliamentary democracy, I think you have got to have parliamentary approval if you have a planned, policy decision to launch a military attack of any significant size.
“To say that parliament is just sidelined before you take such a serious decision is a very retrograde step. It makes parliamentary accountability fairly pathetic.”
French president Emmanuel Macron also laid the ground for action, undertaking a round on interviews in which he said he had proof the Assad regime was responsible for an air strike involving chemical weapons on civilians in Douma.
He explained that he would decide on what action to take once all the information had been verified, insisting he would strive to prevent an escalation of conflict across the Middle East.
Mr Macron has previously called for a “strong” response to last weekend’s attack on the town, for which the Syrian government denies responsibility.
Donald Trump was also meeting his own security officials after a day in which he continued verbose diplomacy over social media.
After earlier having told Russia to “get ready” for missiles to hit Syrian targets he took to Twitter to say an attack on the country “could be very soon or not so soon at all!”.
Just 24 hours earlier he had said on the same platform: “Russia vows to shoot down any and all missiles fired at Syria.
“Get ready, Russia, because they will be coming, nice and new and ‘smart’. You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”
Russia’s ambassador to Lebanon, Alexander Zasypkinhad, had warned that his country would shoot down US rockets, and even fire on “the sources that launched the missiles”.
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