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.
May said the missile strike was created to minimize any civilian casualties and was not an attempt to change the Syrian government. "No other group could have carried this attack", May said.
It said the use of force was permitted providing three conditions were met: evidence of widespread humanitarian distress; no practicable alternative to the use...
"We have sought to use every possible diplomatic channel to achieve this", the prime minister said. One airbase the US fired at was back in use the next day.
May launched the airstrikes without approval from Parliament stating it as national interest and that it was an obligation to deter all those using chemical weapons to kill people.
But she also drew a link with the nerve agent attack on Russian former spy Sergei Skripal and his daughter Yulia in Salisbury.
"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", she said.
The Prime Minister will tell MPs on Monday that the strikes were in the national interest because the use of chemical weapons can not be normalised, including in the UK. Indeed, we have been calling for Parliament to be recalled since last Wednesday.
Yesterday Prime Minister Theresa May insisted the military action was "legal" and defended the decision to go ahead without securing the backing of Parliament.
May is not obliged to win parliament's approval before ordering military action, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the US -led invasion of Iraq.
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".
"Bombing can not substitute for diplomacy", he said. U.S. Congressman Brad Schneider, a Democrat, called for a more comprehensive strategy to deal with Syria at an unrelated panel discussion.
In a statement Mrs May said that the Syrian regime had demonstrated a "persistent pattern of behaviour" when it came to the use of chemical weapons, that "must be stopped".
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