The government is not legally bound to seek Parliament's approval for military strikes, though it is customary to do so.
She was attacked from opposition politicians as well as at least one of her own Conservatives as she gave a report on the weekend air strikes by Britain, the USA and France.
"No other group could have carried out this attack", she told MPs, adding that the Syrian authorities had reportedly attempted to "conceal the facts. supported by the Russians".
"I'm absolutely clear that it is parliament's responsibility to hold me to account for such decisions and parliament will do so", she told the House of Commons in a rowdy session that laid bare divisions over the military action.
French Prime Minister Edouard Philippe also justified the military action in a speech Monday to the National Assembly, France's lower house of parliament.
Ian Blackford, the leader of the opposition Scottish National Party in Westminster, was another of her critics who asked May why she had broken with a convention dating back to the 2003 invasion of Iraq and not recalled parliament for a vote.
May has weathered months of doubt over her leadership due to rows over the Brexit decision to leave the European Union and an ill-judged decision to call an early election when her Conservative Party lost its parliamentary majority. "We have done it because we believed it was the right thing to do", she said.
May called the situation in Syria "a stain on our humanity" and stressed that the humanitarian situation made it "morally and legally right" to take action, as diplomatic efforts had been "repeatedly thwarted" and that diplomacy would not work on its own.
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Theresa May said the United Kingdom government "strongly supports" the mission.
Speaking to the BBC, Johnson said what he described as the successful strikes on three sites in Syria were a message from the world that enough was enough, but acknowledged he could not say whether Assad still had chemical weapons.
"These strikes by these governments, these don't really affect in any tangible way the larger, very painful status quo that Syria is stuck in". May has emphasised that the strikes were "limited" to only target Damascus's chemical weapons programme.
The strikes, carried out alongside the United States, mark May's first major military action since coming to power nearly two years ago.
She says she couldn't wait to act while the Syrian people continue to suffer.
"There's no more serious issue than the life-and-death matter of military action, and Parliament has the right to support or stop the government taking military action".
The poll by Survation for the Mail on Sunday showed 36 percent in favour of Britain's participation in the air strikes, 40 percent against and the remainder undecided.
"She authorised military action with no mandate", said one Conservative lawmaker on condition of anonymity. "If not, she's the one that will take the blame".