Prime Minister Theresa May held an emergency cabinet meeting to discuss joining mooted strikes by the U.S. and allies, with ministers agreeing "on the need to take action", her Downing Street office said in a statement.
Britain is now part of a US-led coalition conducting air strikes against jihadists from the Islamic State (IS) group in Iraq and Syria, and has conducted more than 1,700 such attacks.
The U.S., France and Britain have been consulting about launching a military strike, and President Donald Trump tweeted Wednesday that missiles "will be coming".
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act", May told reporters on Wednesday.
Russian military capabilities if the West strikes Syria
This massacre was a significant escalation in a pattern of chemical weapons use by that very awful regime. One year ago, Assad launched a savage chemical weapons attack against his own innocent people.
Parliament is in recess until Monday, though it could be called back early for an emergency debate.
"Six in ten Brits (61%) say that it should be necessary for Parliament to vote first on whether the United Kingdom takes part in military action in Syria - 18% say it should not be necessary for Parliament to vote on the matter", the pollster wrote on Twitter on the results of its survey.
The mission was only allowed after approval by MPs - they backed military action in Iraq in September 2014, and in Syria a year later, strictly limiting strikes to IS targets. By going to war with Syria, we would also be entering into a proxy war with Russian Federation, which is not only unsafe for Britain but the entire world.
Some lawmakers have expressed reservations about taking military action now, but others have come to believe the 2013 vote was a mistake.
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Many MPs have called for Britain to act against Syria, warning that the use of chemical weapons was in breach of worldwide law and could not be allowed to go unpunished. A YouGov poll showed just one in five members of the public support a strike on Syria.
Attacks by alleged perpetrators since 2013, according to data analysed by Human Rights Watch.
The strike is suspected to be a chemical attack, but the Syrian government denied the allegations saying it was fabricated by the rebels who were in a state of decline.
Trump has since tempered those remarks and the White House said no final decisions on possible actions had been taken. "And what happens after".
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