The foreign secretary has warned that global powers will not “turn a blind eye” to the use of chemical weapons, a day after US-led strikes in Syria.
Writing in the Telegraph, Boris Johnson said countries should know President Assad has not “got away with it”.
It comes after Downing Street published its legal case for its part in the strikes, which targeted military bases.
Sites near Damascus and Homs were hit in response to an alleged chemical attack on the town of Douma.
In the paper, Mr Johnson described the use of toxic weapons as “vile, sick and barbaric” adding it was a “pattern” of behaviour from the Syrian regime.
He said Western powers had tried “countless resolutions” at the UN to deter the use of chemical warfare saying it had become an “effective global taboo”.
The UK believes the Assad regime was responsible for the attack on civilians on 7 April, while the US and France say they have proof.
But Syria has always denied any chemical use and says the attack was fabricated by rebels.
Mr Johnson added: “We are standing up for principle and for civilised values.
“We may not end the barbarism – but we are telling the world that there is one type of barbarism that is banned and that deserves to be banned.”
Prime Minister Theresa May will address Parliament on Monday about her decision as some party leaders have argued MPs should have been consulted before the strikes.
The UK and US have said the strike had been successful with President Trump warning of further action if there are new chemical attacks.
Syrian state media has said it was “a flagrant violation of international law”, while ally Russia’s President Vladimir Putin condemned it “in the most serious way”.
On Saturday, the UN Security Council rejected a resolution drafted by Russia while all Nato allies have given the military action their full support.
There has been no confirmation of any civilian casualties.
What did the UK do?
The Ministry of Defence said eight Storm Shadow missiles had been launched by four RAF Tornados at a former missile base, 15 miles west of Homs.
It is thought President Bashar al-Assad’s regime had been stockpiling materials used to make chemical weapons there, it said.
A spokesperson added the facility was located “some distance” from “concentrations of civilian habitation”, and the risk of contamination to the surrounding area had been minimised.