In a statement released in the early hours of this morning she said: "This evening I have authorised British armed forces to conduct co-ordinated and targeted strikes to degrade the Syrian Regime's chemical weapons capability and deter their use".
Vincent Cable, leader of the opposition Liberal Democrat party, said that while his party did not rule out supporting military strikes, parliamentary approval was essential for any action to be taken.
A statement released by Downing Street after Thursday's Cabinet meeting made no direct reference to military action, but will be seen as a signal Britain would be prepared to join any US-led air strikes against the regime should the Americans decide to go ahead - putting it on a potential collision course with Assad's principal backer Russian Federation.
May said "a significant body of information including intelligence" pointed to Syrian government responsibility for a suspected chemical attack in Douma last Saturday.
Mrs May and the U.S. president discussed the situation on Thursday night, saying there was a need "to deter the further use of chemical weapons by the Assad regime", as they pledged to work together on the worldwide response to the suspected chemical weapons attack.
Mrs May is due to give a Commons statement this Monday before facing questions from MPs.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents", the United States president said in a televised address.
Bombing Syria was in Britain's national interest and has had strong global support, Theresa May will insist as she tells Parliament why she ordered the attack.
U.S. broadcaster NBC quoted USA officials familiar with the intelligence as saying they had now obtained blood and urine samples which had tested positive for chemical weapons.
"This is not about intervening in a civil war".
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Theresa May's Cabinet has agreed Britain needs to take action on the Syrian regime.
"History teaches us that the worldwide community must defend the global rules and standards that keep us all safe".
Trump said the strikes were in coordination with France and the United Kingdom, adding that the goal of the campaign is to "establish a strong deterrent against the production, spread and use of chemical weapons".
Jeremy Corbyn, leader of the main opposition Labour Party, has questioned the legal basis for Britain's involvement.
But she will also ask for an emergency debate to allow more time for discussion in a nod to the fury among MPs at not being consulted.
Russian Federation has blocked moves for a United Nations investigation into the Douma attack and even made the "grotesque and absurd claim that it was staged by Britain", May said. "The primary objective is to say no to the use of barbaric chemical weapons".
Mrs May has faced criticism from across the political spectrum for failing to recall Parliament and put the plans to a vote.
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.
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.
In 2015 lawmakers approved United Kingdom strikes on Islamic State group targets in Syria, but not on government forces.