Prime Minister Theresa May must win MPs’ approval before backing military action in Syria, a motion with cross-party support demands.
Stewart McDonald, the SNP’s Westminster defence spokesman, is pressing for a fresh Parliamentary vote on any airstrikes against Bashar al-Assad regime, and has won the backing of some Labour, Lib Dem and Green MPs.
May sidestepped questions over the issue during a visit to Cambridge on Tuesday after hinting at a press conference earlier in the week the UK could step up its involvement.
America is looking to the UK and France for support as President Donald Trump finalises the US response.
It follows news of a suspected chemical attack by Syrian government forces in a brutal assault on the rebel-held town of Douma that left more than 40 people - including children - dead.
Rescuers and opposition forces said poison gas was used, an allegation strongly denied by the Assad government and his allies in Iran and Russia.
Reports suggested more than 500 people, mostly women and children, were brought to medical centres with difficulty breathing, foaming at the mouth, and burning sensations in the eyes. Families were also reportedly found suffocated in their homes and shelters.
US President Donald Trump described Assad as an “animal” after the attack, and warned there would be a “big price to pay”.
May is set to speak to US President Donald Trump in a phone call on Tuesday night and will be chairing a National Security Council meeting.
Trump and Emmanuel Macron have already agreed to coordinate a “strong, joint response”, but May appears to be more cautious.
“This attack that took place in Douma is a barbaric attack,” she told reporters.
“Obviously we are working urgently with our allies and partners to assess what has happened on the ground.
“If this is the responsibility of Assad’s regime in Syria then it’s yet another example of the brutality and brazen disregard for their people that they show.”
The Prime Minister faces growing pressure from her own MPs to use executive powers to back military action without MPs’ approval, however.
Tom Tugendhat, chair of the Foreign Affairs Select Committee, said: “Striking Syria’s stockpiles of chemical weapons would degrade their ability to commit further war crimes and could be done together with allies. It would not require a vote in Parliament.”
Dominic Grieve, the former attorney general, has also thrown his weight behind an attack, writing in his Evening Standard column: “In view of the enormity of what has been done, if there is a way of preventing its recurrence by the limited use of force that is one thing and I would be supportive.”
Former Labour Prime Minister Tony Blair has also called on Britain to support US airstrikes, saying “non-intervention is also a policy with consequences”.
He told BBC Radio 4’s Today programme: “If the US are taking action, we should be prepared to be alongside them.
“Last year they took military action and that was supposed to deter these attacks. It obviously had some impact, but now we have another one of those attacks.”
But choosing to act without Parliament’s support could be divisive.
McDonald’s motion has the backing of 46 MPs so far, including that of Labour’s Clive Lewis, the Greens’ Caroline Lucas and Lib Dem Jo Swinson.
Labour leader Jeremy Corbyn is not thought to support military action and his party has been widely condemned for a lacklustre response.
In a statement which failed to apportion blame, Labour said that “anyone found responsible for using chemical weapons is brought to justice”.
The statement said Syrian people had “suffered for too long from the atrocities and brutality of this war, whether committed by the Assad regime, by Jihadist militias, or by their respective international supporters”.
Some Labour MPs are likely to back action, however.
Trump did not give a time frame for any retaliatory action, but said the US could not stand by as such atrocities take place because “we are able to stop it”.
Tensions between western leaders and Russia, which supports Syria’s actions, have meanwhile hit boiling point.
Moscow’s ambassador to the UN, Vassily Nebenzia, warned of “grave repercussions” of US airstrikes during heated exchanges at the UN Security Council.
US ambassador Nikki Haley accused Russia of having “the blood of Syrian children” on its hands after Trump said that “nothing’s off the table” in dealing with the alleged outrage.
Foreign Secretary Boris Johnson said in February that Britain should consider joining military action against Assad’s regime if there is fresh “incontrovertible” evidence he has used chemical weapons against his own people.
The attack in Douma occurred late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
The attack comes almost exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base.