Theresa May is braced for a Commons showdown after bowing to pressure and allowing a debate in parliament on the UK’s role in air strikes against the Syrian regime.

She will make a statement on the British, French and US operation that saw more than 100 missiles fired at Syria, before being grilled by MPs who were denied a vote ahead of the action.

Ministers hope the six-hour emergency debate will pacify concerns that parliament is being sidelined, but are desperate to avoid allowing any substantive vote that risks stripping the operation of legitimacy.

It sets the stage for an opposition-party drive to force a more meaningful retrospective vote on Saturday’s action in the coming days, with Conservative MPs given strict orders to be available for voting on both Monday and Tuesday.

Labour leader Jeremy Corbyn went even further on Sunday and demanded a new “War Powers Act” that would see all future prime ministers requiring MPs’ approval before taking almost any military action.

In her statement to parliament on Monday, Ms May will try and flesh out the legal and moral case for her decision to send British fighter jets into action, arguing that she sought to alleviate further humanitarian suffering.

She will highlight the operation’s broad international backing, claim it was in the national interest and criticise Russia for manoeuvres at the UN to block investigations into the Douma chemical weapons incident that galvanised the West to act.

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Ms May will say: “We are confident in our own assessment that the Syrian regime was highly likely responsible for this attack and that its persistent pattern of behaviour meant that it was highly likely to continue using chemical weapons.”

Linking the attack to the UK’s broader stand-off with Russia over the Salisbury nerve agent poisoning, she will add: “It is in our national interest to prevent the further use of chemical weapons in Syria and to uphold and defend the global consensus that these weapons should not be used.

It is in our national interest to prevent the further use of chemical weapons in Syria and to uphold and defend the global consensus that these weapons should not be used Prime Minister Theresa May

“For we cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere.”

Her emphasis on the national interest may also speak to polls, including one published by The Independent, showing minimal public support for the Syria strikes.

In the immediate aftermath of the weekend’s action, the prime minister said that by convention it was her “prerogative” to take military action without parliamentary approval. But with pressure over the issue growing she appears to have judged it wise to allow MPs sound off.

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The emergency debate has been called under procedures laid out under House of Commons Standing Order 24, which allows for a general debate.

While it will give MPs the chance to air their views and make prolonged arguments and criticism of the government, it will only be accompanied by a neutral motion, such as “that the House has considered the matter of military action taken on 14 April”.

It means that even if opposition MPs demand a vote on the motion, it will not necessarily affect the legitimacy of the air strikes if the government loses.

The Independent understands that opposition parties are considering pushing it to a vote anyway, simply to make the point that some sort of vote should be held.

Labour and other parties may also try and force their own emergency debates on Tuesday that could carry with them more substantive motions on whether the Commons should back the strikes, which if lost would create serious problems for the prime minister’s credibility.

If it comes to a vote, the parliamentary arithmetic will be finely balanced for Ms May – given a handful of Tory MPs, including Zac Goldsmith and Julian Lewis, have previously demanded the Commons be allowed to vote prior to action.

Syria bombings: US, UK and French military air strikes wreckage Show all 13 1 /13 Syria bombings: US, UK and French military air strikes wreckage Syria bombings: US, UK and French military air strikes wreckage The wreckage of the Scientific Studies and Research Centre in the Barzeh district, north of Damascus, which was targeted by the US, UK and France air strikes. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage of the Syrian Scientific Research Center surrounded by papers and rubble. AP Syria bombings: US, UK and French military air strikes wreckage Firefighrers extinguish smoke that rises from the damage. The Pentagon says none of the missiles filed by the U.S. and its allies was deflected by Syrian air defenses, rebutting claims by the Russian and Syrian governments. AP Syria bombings: US, UK and French military air strikes wreckage The wreckage of part of the Scientific Studies and Research Centre compound . AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Part of a building collapsing, surrounded by the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damaged to the Scientific Studies and Research Centre compound. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Lt. Gen. Kenneth McKenzie, the director of the Joint Staff at the Pentagon, also says there also is no indication that Russian air defense systems were employed early Saturday in Syria. AP Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier sprays water on the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Syrian state news agency SANA reported several missiles hit a research centre in Barzeh, north of Damascus, "destroying a building that included scientific labs and a training centre". AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage. AP Syria bombings: US, UK and French military air strikes wreckage Damage to the Scientific Research Center building that was hit by the strikes. EPA Syria bombings: US, UK and French military air strikes wreckage The Scientific Studies and Research Centre was one of the targeted buildings by the US, UK and France. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damage to the centre. EPA

But there are also between 30 and 50 Labour rebels who might give the government’s operation their backing, while at least some of the Tory MPs will be happy that Monday’s debate is enough to ensure parliament has had its say.

One Conservative who previously had concerns told The Independent: “I think the government can make a reasonable case that it acted in a restricted way and engaged with all sorts of caveats, that this was not involvement in the civil war and was not regime change.”

In a bid to allay concern that the government may already be planning further action against the Assad regime, foreign secretary Boris Johnson said there was “no proposal on the table at the moment”.

But he added: “If and when [further use of chemical weapons] were to happen, then clearly, with allies, we would study what the options were.”

Syria: Jeremy Corbyn calls for War Powers Act to limit government's ability to launch air strikes without asking MPs first

Mr Corbyn and his shadow foreign secretary Emily Thornberry both said on Sunday that parliament should have been given a vote ahead of strikes, which saw four RAF Tornados fire Storm Shadow missiles at a chemical weapons facility.

The Labour leader went on to say: “I think what we need in this country is something more robust like a War Powers Act so that governments do get held to account by parliament for what they do in our name.”

He warned of an escalation in a “proxy war” between the US and Russia and argued that chlorine, said to have been used in the Douma attack, has also been used by “a number of parties in the conflict” in Syria as a weapon.

Tony Blair’s push for war in Iraq in 2003 was the first time such a decision was put to a vote in the Commons.

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In 2011 David Cameron offered no vote before UK air strikes on Libya, but won a retrospective one shortly after, once British service personnel were committed.

But then in 2013 he lost a vote to take action against Assad in Syria, winning votes the following two years to take limited action against Isis in Iraq and Syria.

Scotland’s first minister Nicola Sturgeon said it was a “serious mistake” for the role of UK armed forces in Syria to be altered without the approval of parliament.