In 2013, Britain did not take part in military action in response to a chemical attack in Syria after then Prime Minister David Cameron failed to get approval from Parliament. May said she would be addressing parliament on Monday.
He posted on Twitter: "Never said when an attack on Syria would take place".
Protesters hold up a placard as bus passengers look on during a demonstration, Friday, April 13, 2018, in London, organised by the Stop the War Coalition against possible military intervention or bombing by western allies in Syria.
The tornadoes launched storm shadow missiles at a military facility some 15 miles west of Homs, where the Assad regime was assessed to have kept chemical weapons in breach of Syria's obligations under the Chemical Weapons Convention.
She told reporters during a visit to Birmingham that she was "appalled" but "not surprised" by Russia's decision to veto a draft resolution at the United Nations on Tuesday which sought to create a new body to determine responsibility for the attack.
"I believe that the action taken will have significantly degraded the Syrian regime's ability to use chemical weapons", she said.
"Britain's Ministry of Defence said four Tornado jets using Storm Shadow missiles had taken part in the attack which had been created to target a military facility where it was assessed Syria had stockpiled chemicals". "And we will be working with our closest allies on how we can ensure that those who are responsible are held to account and how we can prevent and deter the humanitarian catastrophe that comes from the use of chemical weapons in the future".
The Cabinet agreed with the Prime Minister's assertion it is "highly likely" President Assad's regime carried out a "shocking and barbaric" suspected gas attack in the Syrian city of Douma on Saturday.
"The red line set by France in May 2017 has been crossed", he said.
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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.
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.
"The facility which was struck is located some distance from any known concentrations of civilian habitation, reducing yet further any such risk".
Missiles streak across the Damascus skyline as the U.S. launches an attack on Syria targeting different parts of the capital, early on Saturday.
May is in a "lose-lose situation", with the suspected nerve agent attack on former Russian spy Sergei Skripal in Wiltshire last month further complicating matters, says The Times's Matt Chorley.
"There is no graver decision for a prime minister than to commit our forces to combat and this is the first time I have had to do so", she said. This action is absolutely in Britain's national interest.
A team from the Organisation for the Prohibition of Chemical Weapons is due to start its investigation in Syria on Saturday.
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.
The Cabinet meeting suggests May could decide to sidestep parliamentary approval for military strikes amid reports that Assad has begun moving his arsenal away from air bases that are likely to be targeted.