"This is the first time as prime minister that I have had to take the decision to commit our armed forces in combat -- and it is not a decision I have taken lightly".
The Prime Minister is expected to face angry MPs after launching military action without securing the support of the Commons.
Lawmakers backed action in Iraq in 2014, and again in Syria in 2015, strictly limiting strikes in both countries to targets of the Islamic State jihadist group.
May has repeatedly said that the missile strike on Syria was not about "regime change".
A YouGov poll in The Times conducted this week found that 43 percent of voters opposed strikes in Syria, with 34 percent unsure and only 22 percent supportive.
Stop the War, a pacifist coalition once chaired by Corbyn, has called a demonstration outside the British parliament on Monday to protest against the strikes. "And that to me is not a good enough reason".
The Labour leader said chlorine has been used by "a number of parties in the conflict" in Syria as a weapon and questioned the legality of the airstrikes.
"The Government's own justification, which relies heavily on the strongly contested doctrine of humanitarian intervention, does not even meet its own tests".
The prime minister added the military assault was not about regime change, toppling Assad or intervening further in the war.
Whitehall sources have confirmed a 20-fold increase in "disinformation" spread by Kremlin-linked social media "bot" accounts since the strikes, according to The Daily Telegraph.
Russian Federation has warned the West against attacking its Syrian ally President Bashar al-Assad, who is also supported by Iran, and says there is no evidence of a chemical attack in Douma, a town near Damascus which had been rebel-held until this month.
"We can not allow the use of chemical weapons to become normalized - within Syria, on the streets of the United Kingdom, or anywhere else in our world".
She will say: "Let me be absolutely clear: we have acted because it is in our national interest to do so".
The British, US and French bombings on Saturday followed an alleged chemical weapons attack on the rebel-held town of Douma on April 7.
Germany, Italy, Canada, Australia and European Council President Donald Tusk have "all have expressed their support for the actions that Britain, France and America have taken", the PM will add.
May has said "all indications" point to Syrian responsibility for the attack.
May said the aim was to deter the Syrian authorities from further use of chemical weapons and to send a message to the wider world that it was unacceptable to use such weapons.
"Furthermore, there were clearly attempts to block any proper investigation, as we saw with the Russian veto at the United Nations earlier in the week".