The Cabinet is expected to be asked to approve a form of British participation in action led by France and the US, aimed at Syrian President Bashar al Assad’s chemical weapons infrastructure.
It comes after a suspected chemical attack on the Syrian city of Douma, eastern Ghouta, reportedly killed 70 people and injured 500 at the weekend.
Importantly, it appears Parliament will not be consulted ahead of the UK action, which could start within hours of Cabinet approval.
That will prove controversial, after opposition parties demanded the Government respects a recent convention that the House of Commons approves UK military intervention.
Labour leader Jeremy Corbyn said on Wednesday that MPs “should always be given a say on military action”.
“That’s a case that I’ve made going back many, many years in Parliament,” he added.
“Obviously the situation is very serious, obviously there has to be, now, a demand for a political process to end the war in Syria.
“We cannot risk an escalation even further than it’s gone already”.
Liberal Democrat leader Sir Vince Cable said President al Assad had crossed “a clear red line”, but also demanded the Government “present the objectives of any proposed action to Parliament”.
“A unilateral response by any country, outside of a wider strategy, without allies, is not the way forward,” he said.
“There must be a debate and vote in the House of Commons ahead of any military action.”
Meanwhile, SNP defence spokesman Stewart McDonald wrote to the Prime Minister demanding a parliamentary vote on possible action, writing: “Any proposed change to the role of UK forces in Syria must be subject to full scrutiny and a vote in Parliament.”
A number of Conservative MPs also urged some caution as they cited a lack of clarity on strategic objectives.
Sir David Amess, speaking to Sky News, implored Mrs May to follow Winston Churchill’s maxim of “jaw jaw” being preferable to “war war”.
Fellow Tory MP Bob Seely, a member of the House of Commons’ Foreign Affairs Committee and a supporter of action in Syria the previous time MPs voted in 2013, is now a sceptic.
He warned that without a strategy the action was “gesture bombing” and would be “really dangerous”.
Conversely, Sarah Wollaston, who voted against action to deter chemical weapons use by President al Assad in 2013, supported the pro-intervention approach of younger Conservative MPs such as Foreign Affairs Select Committee chair Tom Tugendhat and former army officer Johnny Mercer.
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A senior Tory revealed to Sky News the Prime Minister cannot be certain of getting the numbers, in any vote, and needed to tread carefully on the legal underpinning of any intervention.
But, the direction of travel of Government policy is that Mrs May will seek Cabinet pre-approval for this new military involvement, rather than from the House of Commons, therefore backing away from a convention set since the controversial 2003 Iraq War.