The Prime Minister will tell MPs she acted "in Britain's national interest" by ordering airstrikes on Syria as the Government calls for an emergency debate on the issue.
Theresa May will pre-empt planned opposition motions by applying to the Speaker for a debate herself "to give the House an extended opportunity to discuss the military action".
In a statement to the Commons, she will also set out her justification for the decision – arguing that it was done to alleviate further humanitarian suffering in Syria caused by chemical weapons attacks.
The PM will say: "UNSC-mandated inspectors have investigated previous attacks and on four occasions decided that the regime was indeed responsible.
"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.
"Furthermore, there were clearly attempts to block any proper investigation, as we saw with the Russian veto at the UN earlier in the week.
"And we cannot wait to alleviate further humanitarian suffering caused by chemical weapons attacks.
"Let me be absolutely clear: we have acted because it is in our national interest to do so.
"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.
"For we cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere."
A source close to Labour leader Jeremy Corbyn has described the Prime Minister's announcement of an emergency debate as "very odd, panicky and weak".
They argue the Government has been trying to block the debate all weekend and, as the party in government, has the power to set Government business without asking the Speaker for an emergency debate.
"They are in government. They can set the agenda," said the source.
Mr Corbyn will also call for a War Powers Act which would oblige prime ministers to consult Parliament in the future before committing forces into the field of action.
The SNP has told Sky News it would support the act.
In a statement, the party says there are fresh questions over the Government's legal justification and the need to allow the OPCW (Organisation for the Prohibition of Chemical Weapons) to fully investigate the chemical attacks in Douma last weekend.
The SNP's Westminster leader, Ian Blackford, said: "It is welcome that the Prime Minister has finally caved in to our calls for a full parliamentary debate on the escalation of military action in Syria.
"But a debate and vote should have happened last week before any change to the role of UK forces in Syria. Indeed, we have been calling for Parliament to be recalled since last Wednesday.
"This extraordinary u-turn is an admission that the Prime Minister made an error in failing to recall Parliament and is yet more evidence of how ill considered this military action is – and just how far Theresa May's actions have been dictated by presidential tweet.
"A clear precedent was set in 2015 ahead of the targeted strikes against Daesh – yet the Prime Minister decided to ignore this before launching airstrikes.
"Theresa May needs to realise that she leads a minority government and that Parliament must have a voice in matters of such importance before actions like that are taken.
"This debate is an opportunity to ensure that there is no future change in the role of British military in Syria without Parliament's approval."
But in her statement the Prime Minister will point to the international backing which the UK and its allies have since received.
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Theresa May will say: "Over the weekend I have spoken to a range of world leaders, including Chancellor Merkel, Prime Minister Gentiloni, Prime Minister Trudeau, Prime Minister Turnbull and European Council President Donald Tusk.
"All have expressed their support for the actions that Britain, France and America have taken."