Theresa May is to hold an emergency cabinet meeting to discuss joining the United States and France in possible military action against Syria.
The prime minister summoned her senior ministers to No 10 after saying “all the indications” were the regime of Bashar al-Assad was responsible for a suspected chemical weapons attack on civilians last weekend.
Donald Trump warned Russia of imminent military action in Syria, declaring missiles “will be coming” and denouncing Moscow for standing by Syria’s president.
It followed the Kremlin’s insistence that it would shoot down any missiles and attack their source, in developments that have placed the two global and nuclear superpowers closer to open conflict than at any time since the Cold War.
Russia has repeatedly warned the West against attacking its Syrian ally, which is also supported by Iran.
Moscow has said there was no chemical attack in Douma, in the besieged rebel enclave of eastern Ghouta, near the capital Damascus.
Ms May recalled the ministers from their Easter holiday for a special cabinet meeting in Downing Street to discuss Britain’s response to what she has cast as a barbaric attack which cannot go unchallenged.
“The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act,” Ms May said. “All the indications are that the Syrian regime was responsible.”
The Organisation for the Prohibition of Chemical Weapons (OPCW) had said it intends to send investigators to Douma to look for any evidence of a chemical attack.
Ms May has faced calls to wait for unequivocal proof of a chemical attack by the Assad regime before committing British forces to retaliatory action.
The Labour leader, Jeremy Corbyn, joined other opposition parties, as well as some Conservative backbenchers, in insisting MPs must be have a say on any British involvement in military action.
However, Ms May faces growing impatience from Washington, after Mr Trump’s tweet to say the missiles “will be coming”.
The prime minister is not obliged to win parliament’s approval, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the US-led invasion of Iraq.
It has been observed in subsequent military deployments in Libya and Iraq.
Britain has been launching air strikes in Syria from its military base in Cyprus, but only against targets linked to Isis.
Parliament voted down British military action against Mr Assad’s government in 2013, in an embarrassment for David Cameron.
The vote deterred Barack Obama’s administration from similar action.