The PM will say that United Nations Security Council-mandated inspectors have probed previous attacks and decided Bashar Assad's regime was responsible four times.
Mrs May will make the statement explaining why she ordered British cruise missile strikes and will then take questions from MPs.
She said: "In Douma, last Saturday a chemical weapons attack killed up to 75 people, including young children, in circumstances of pure horror". It is not about regime change.
At least six loud explosions were heard in Damascus, and smoke was seen rising over the city even while Trump delivered his remarks.
"We can not allow the use of chemical weapons to become normalised - within Syria, on the streets of the United Kingdom, or anywhere else in our world", she said.
Prime Minister Theresa May said there was "no practicable alternative to the use of force" to deter the use of chemical weapons by the Syrian regime.
The Western missile strikes demonstrate the volatile nature of the Syrian civil war, which started in March 2011 as an anti-Assad uprising but is now a proxy conflict involving a number of world and regional powers and a myriad of insurgent groups.
French President Emmanuel Macron said the strikes had been limited so far to Syria's chemical weapons facilities.
"The facility which was struck is located some distance from any known concentrations of civilian habitation, reducing yet further any such risk", the MoD said in a statement.
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But she also drew a link with the nerve agent attack on Russian former spy Sergei Skripal and his daughter Yulia in Salisbury.
Tonight I want to speak with you about why we have taken this action.
"I have done so because I judge this action to be in Britain's national interest", she added.
Over 100 missiles were fired on the morning of Saturday, April 14, targeting what representatives of coalition forces called chemical weapons sites in retaliation for an earlier poison gas attack.
"A clear precedent was set in 2015 ahead of the targeted strikes against Daesh - yet the Prime Minister made a decision to ignore this before launching airstrikes".
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".
"This collective action sends a clear message that the global community will not stand by and tolerate the use of chemical weapons", May said at a press conference, calling the military action "right and legal".
"Bombing can not substitute for diplomacy", he said.
"One year ago, Assad launched a savage chemical weapons attack against his own innocent people".
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. "We believe that the strikes we have taken last night had a significant impact in terms of what the Syrian regime can do in the future".