Four Royal Air Force Tornado jets using Storm Shadow missiles had taken part in the attack on a military facility near Homs where it was assessed Syria had stockpiled chemicals, Britain's Ministry of Defence (MoD) said.
Britain joined the United States and France in what May cast as a "limited and targeted" strike after intelligence indicated Syrian President Bashar al-Assad's government was responsible for an attack using chemical weapons in Douma last Saturday.
May said the missile strike was designed to minimise any civilian casualties and was not an attempt to change the Syrian government.
"This is not about intervening in a civil war. It is not about regime change," May said in a statement.
Britain's defence ministry said initial indications were that the precision weapons and meticulous target planning had "resulted in a successful attack".
May said Britain and the West had an obligation to deter both Assad and others from using chemical weapons after the poison gas attack in Douma near Damascus killed up to 75 people including children.
May added Britain and its allies had sought to use every diplomatic means to stop the use of chemical weapons, but had been repeatedly thwarted, citing a Russian veto of an independent investigation into the Douma attack at the U.N. Security Council this week.
"So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime," she said.
The Western missile strikes demonstrate the volatile nature of the Syrian civil war, which started in March 2011 as an anti-Assad uprising but is now a proxy conflict involving a number of world and regional powers and a myriad of insurgent groups.
U.S. President Donald Trump said he was prepared to sustain the response until the government of Assad stopped its use of chemical weapons.
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.
CHEMICAL WEAPONS TARGETS
Britain's defence ministry said "very careful scientific analysis" had been applied to maximise the destruction of stockpiled chemicals while minimising any risk of contamination to surrounding areas.
May said while the strike was targeted at Syria, it sent a message to anyone who used chemical weapons. Britain has accused Russia of being behind last month's nerve agent attack on former Russian spy Sergei Skripal and his daughter in Salisbury, southern England, a charge Moscow has rejected.
"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," she said.
"I have done so because I judge this action to be in Britain's national interest. We cannot allow the use of chemical weapons to become normalised - within Syria, on the streets of the UK, or anywhere else in our world."
Many politicians in Britain, including some in May's own Conservative Party, had called for parliament to be recalled from a break to give authority to any military strike.
Former Prime Minister David Cameron lost a parliamentary vote on air strikes against Assad's forces in 2013 when 30 Conservative lawmakers voted against action, with many Britons wary of entering another conflict after intervention in Iraq, Afghanistan and Libya failed to bring stability to the region.
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 U.S.-led invasion of Iraq.
A YouGov poll for The Times newspaper this week indicated that only a fifth of voters believed that Britain should launch attacks on Syrian military targets and more than two-fifths opposed action.
Opposition Labour leader Jeremy Corbyn had said Britain should press for an independent U.N.-led investigation into the suspected chemical attack in Douma rather than wait for instructions from Trump on how to proceed.
Former Labour foreign secretary David Miliband, now head of humanitarian relief group the International Rescue Committee, said military action needed to be part of a wider political strategy.
"Bombing cannot substitute for diplomacy," he said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections. Election results will be out on May 23.