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.
"This is not about intervening in a civil war".
"It was not about regime change".
DUP leader Arlene Foster said the air strikes were "limited but proportionate and justified".
May said intelligence and open source accounts indicated that the regime was behind the attack in Douma last Saturday.
"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".
May will face questions from MPs on Monday, when parliament reconvenes after a break.
Liberal Democrat leader Vince Cable said that "riding the coat-tails of an erratic U.S. president is no substitute for a mandate from the House of Commons", parliament's elected lower house.
Britain has blamed Russian Federation for the poisoning - a charge vehemently denied by Moscow which has accused London of failing to come up with evidence for its claims.
The ministry said the facility was "a former missile base... where the regime is assessed to keep chemical weapon precursors".More news: Marathon runner Callum Hawkins collapses in final kilometres of Commonwealth Games race
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The group said it "strongly condemned" the action and accused May of "sanctioning killing" at US President Donald Trump's behest.
"The facility which was struck is located some distance from any known concentrations of civilian habitation, reducing yet further any such risk", it added. As the Conservative leader explained her rationale for the air strikes, opposition parties claimed the attacks were legally dubious, risked escalating conflict and should have been approved by lawmakers.
Parliament is not due to reconvene until Monday, following its Easter recess.
The shadow of the 2003 invasion of Iraq still lingers in the corridors of Britain's parliament, when MPs backed then-prime minister Tony Blair in joining United States military action.
A YouGov poll of 1,600 British people, published on Thursday, found that only 22% supported military action against Syria, while 43% opposed it.
He also did not rule out future military action.
They also discussed the need to reinvigorate multinational stabilization efforts in Syria to ensure the long-term defeat of Islamic State (IS). "The opposition does not operate helicopters or use barrel bombs".
"This action risks not just further escalating the civil war in Syria but also a risky escalation of worldwide tensions", said the leader of the left-wing Scottish National Party, the third-biggest force in the British parliament.
When asked if Syria's Assad could remain leader as long as he refrained from further use of chemical weapons, May said: "This was about, as I have said and you have recognized, this was specifically about the use of chemical weapons".
Labour MP Kevin Brennan wrote: "Parliamentary approval should have been sought".