U.S., British, and French forces launched air strikes against targets in Syria in the first coordinated Western military action against the Damascus regime.
May said the missile strike was created to minimize any civilian casualties and was not an attempt to change the Syrian government.
She declined to say whether Bashar al-Assad should stay in power and said talks with allies would continue on finding a political solution to the civil war. "It is not about regime change", May said in a statement.
May said intelligence and open source accounts indicated that the regime was behind the attack in Douma last Saturday.
Downing Street yesterday published the government's legal position, which said the attacks were permitted under global law after the deaths of 400,000 people in the conflict.
In separate calls, the Prime Minister spoke to the U.S. and French presidents to discuss the offensive and the three leaders agreed it had been a "success", a No 10 spokesman said.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents", the USA president said in a televised address.
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. Russian Federation and Syria claim the attack was fabricated.
"Very careful scientific analysis was applied to determine where best to target the Storm Shadows to maximise the destruction of the stockpiled chemicals and to minimise any risks of contamination to the surrounding area", the ministry said.
May said the strikes would "send a clear signal to anyone else who believes they can use chemical weapons with impunity".
She added that she would address parliament on Monday.
"I have done so because I judge this action to be in Britain's national interest", she added.
"This extraordinary u-turn is an admission that the Prime Minister made an error in failing to recall Parliament and is yet more evidence of how ill considered this military action is - and just how far Theresa May's actions have been dictated by presidential tweet".
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.
He also did not rule out future military action.
Opposition Labour leader Jeremy Corbyn had said Britain should press for an independent United Nations -led investigation into the suspected chemical attack in Douma rather than wait for instructions from Trump on how to proceed.