"There is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime", May said in a televised statement.
"This collective action sends a clear message that the global community will not stand by and tolerate the use of chemical weapons", May told a press conference.
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 USA military action.
A YouGov poll in The Times conducted this week found that 43 percent of voters opposed strikes in Syria, with 34 percent unsure and only 22 percent supportive.
"The government should do whatever possible to push Russian Federation and the United States to agree to an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account", he added.
It was the first time parliament had voted against a British government taking military action.
May said "a significant body of information including intelligence" pointed to Syrian government responsibility for a suspected chemical attack in Douma last Saturday.
Jeremy Corbyn, leader of the main opposition Labour Party, had on Friday accused the government of "waiting for instructions" from US President Donald Trump on what to do over Syria.
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Shortly after the military strikes were launched, Scotland's first minister Nicola Sturgeon said United Kingdom foreign policy should be set by Parliament and not Donald Trump after the U.S., United Kingdom and France bombed targets in Syria.
"Theresa May should have sought parliamentary approval, not trailed after Donald Trump", Corbyn said.
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.
Following the air strikes, there was criticism from some opposition MPs.
Britain's defence ministry said in a statement that four British Tornado jets had fired Storm Shadow missiles at the base 15 miles (24 kilometres) west of Homs at 0100 GMT.
It is reported that in addition to the RAF's Storm Shadow projectiles, French SCALP-EG and the American JASSM were also likely used in the strike, as well as Tomahawk cruise missiles.
"There must be urgent confirmation from the prime minister that there will be no further action. without a full parliamentary debate".
Professor Iain Begg, Research Fellow at the European Institute and Co-Director of the Dahrendorf Forum at the London School of Economics and Political Science (LSE), told Xinhua: "A volley of bombs may help the US and its allies feel they have reacted in a timely and proportionate manner to the undoubted horror of the use by the Syrian regime of chemical weapons, but the inevitable worry will be that they have not thought through what happens next".
May's office said she had spoken to the Crown Prince of Saudi Arabia, Mohammed bin Salman, Turkish President Tayyip Erdogan, German Chancellor Angela Merkel and the prime ministers of Italy, Australia and Canada about the strikes.