"This collective action sends a clear message that the global community will not stand by and tolerate the use of chemical weapons", May said at a press conference, calling the military action "right and legal".
Speaking from the White House on Friday, president Donald Trump announced the military action, and said its intention was to degrade Syrian chemical weapons capabilities.
And a significant body of information including intelligence indicates the Syrian Regime is responsible for this latest attack.
May said the strikes would "also send a clear signal to anyone else who believes they can use chemical weapons with impunity".
She said the strikes were "right and legal" to "prevent further humanitarian crisis".
The Prime Minister defended her Government's decision to carry out air strikes in Syria, claiming the air strikes were in Britain's national interest. Syria and its main ally Russian Federation deny a chemical attack took place. It is not about regime change.
He warned that intervention would lead to a proxy war with Russian Federation which would be "not only unsafe to Britain, but the entire world".
Russia, which intervened in the war in 2015 to back Assad, has denied there was a chemical attack and has accused Britain of helping to stage the Douma incident to stoke anti-Russian hysteria.
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Correspondents from the British and world press also asked the prime minister to explain what role can the Parliament play in making a decision to attack another sovereign country.
May held an emergency cabinet meeting to discuss possible action on Thursday and there had been calls for the British parliament to be consulted before any air strikes.
Britain's defence ministry said in a statement that four British Tornado jets had fired Storm Shadow missiles at the Syrian base 24 km west of Homs at 0100 GMT.
Former PM David Cameron, who lost a vote in 2013 on taking action in Syria, said Mrs May was right to take 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.
"This legally questionable action risks escalating further.an already devastating conflict", he said, adding that May should have sought parliamentary approval.
He tweeted: "I firmly support the military action taken in Syria".
Scottish First Minister Nicola Sturgeon also said the strikes risked "dangerous escalation".