- Video report by ITV News Correspondent Angus Walker
The Cabinet has "agreed on the need to take action" in response to a "highly likely" chemical weapon attack by Syrian forces.
The meeting at Number 10 was held to determine whether Britain would join US-led air strikes against the regime of president Bashar Assad.
Meanwhile, US President Donald Trump has emerged from a meeting with his national security team without a "final decision" on how to respond to Syria.
White House spokeswoman Sarah Huckabee Sanders said Mr Trump held a meeting with his team to discuss the situation but "no final decision has been made".
She said that US officials are "continuing to assess intelligence" and are "engaged in conversations with our partners and allies".
Downing Street said the Prime Minister had spoken to President Trump on the telephone about Syria on Thursday night.
- Video report by ITV News Washington Correspondent Robert Moore
Downing Street said in a statement: "They agreed that the Assad regime had established a pattern of dangerous behaviour in relation to the use of chemical weapons.
"They agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime.
"They agreed to keep working closely together on the international response."
What did the Cabinet agree?
A statement said ministers concluded it is "vital that the use of chemical weapons did not go unchallenged" and would continue to work with France and the US to "coordinate an international response".
The aim of action is to "alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime".
ITV News Political Editor Robert Peston noted the Cabinet statement gave "no guidance on timing other than that a co-ordinated plan has not yet been agreed".
There are currently over a dozen British fighter jets stationed at RAF Akrotiri, a location some 315 miles (510km) from Syria, that are likely to be used if allied strikes against the Assad regime go ahead.
What are the UK's military options?
ITV News Correspondent Angus Walker says all the options on the table are "risky."
He said: "Airstrikes could quickly escalate into a war with Syria's allies Russia and Iran and not taking any action would set a dangerous precedent, that chemical weapons, illegal chemical weapons could be used without a united western response."
Will MPs vote on the action?
Mrs May is coming under pressure from some of her own MPs, as well as opposition politicians, to ensure a vote is held in Parliament before any military action is sanctioned.
Labour leader Jeremy Corbyn has called for a "political solution" to the war in Syria, rather than adding to the violence, saying: "More bombing, more killing, more war will not save life."
Mr Corbyn said: "Parliament must be consulted on this. Surely the lessons of Iraq, the lessons that came there from the Chilcott Report, are that there's got to be, there has to be a proper process of consultation."
He added: "We elect Parliament, we elect Members of Parliament. They should have a voice in this. Cabinet on its own should not be making this decision," he added.
Liberal Democrat leader Sir Vince said while the use of chemical weapons crossed a "clear red line", ministers should present their evidence of regime involvement to MPs.
How has the government responded to the calls?
Downing Street would not be drawn on claims Mrs May was preparing to authorise UK forces to strike against Assad without first seeking vote in Parliament - a move Corbyn has condemned.
Ministers were summoned to number 10 after the prime minister said "all the indications" were that the Syrian regime was responsible for an alleged chemical attack on its own people last weekend.
Douma, the site of that attack, is now under Syrian regime control according to the Russian military.
The Organisation for the Prohibition of Chemical Weapons said a special fact-finding mission was on its way to Syria to investigate the suspected chemical attack.