Air strikes on Syria were legally justified on humanitarian grounds, government documents say.
Downing Street published its legal case for military action hours after Theresa May said she was “confident” the strikes carried out by the UK, France and the US had been successful.
Labour leader Jeremy Corbyn described the action as “legally questionable”.
Bases near Damascus and Homs were targeted in response to an alleged chemical attack on the town of Douma.
Syrian state media said the military action had been “a flagrant violation of international law”, while ally Russia’s President Vladimir Putin condemned it “in the most serious way”.
US President Donald Trump hailed the overnight strikes as “perfectly executed”, adding: “Mission Accomplished”.
At a meeting of the UN’s Security Council, the UK’s ambassador Karen Pierce said the strikes had been “limited, targeted and effective”, with the impact on civilians “minimised”.
She said repeated attempts to hold the Syrian regime to account had been met with Russian “obstruction and resistance”, and the UK and its allies would “continue to pursue a diplomatic resolution to the crisis”.
The US’s ambassador Nikki Haley told the council diplomacy had been given “chance after chance”, and the US remained “locked and loaded”.
Britain, France and the US were also due to brief Nato allies on the strikes.
Speaking in Downing Street earlier, Mrs May said: “This collective action sends a clear message that the international community will not stand by and tolerate the use of chemical weapons.”
Drawing a link with the recent nerve agent attack in Salisbury, Mrs May added: “We cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere.”
Mrs May will make a statement in Parliament on Monday and give MPs a chance to ask questions.
Mr Corbyn said MPs should have been consulted before the strike and called on the PM to “publish in full the legal justification and basis for” the action.
Media captionJeremy Corbyn wants the PM to publish the legal justification for air strikes
In the summary of legal advice given by Attorney General Jeremy Wright to Mrs May, Downing Street outlined why the UK was “permitted under international law” to take measures in the face of “overwhelming humanitarian suffering”.
It said there were three reasons why the strikes met international laws for military action on humanitarian grounds:
The “repeated lethal use of chemical weapons by the Syrian regime constitutes a war crime” and it was “highly likely the regime would seek to use” such weapons again.
Other attempts to “alleviate the humanitarian suffering caused by the use of chemical weapons” had been blocked and there was “no practicable alternative” to the strikes.
The action was “carefully considered” and the “minimum judged necessary for that purpose”.
Whitehall officials say the aim of launching strikes against Syria has been to deal a big enough blow to the Assad regime that it deters it from using chemical weapons again – but not so big as to alter the course of the Syrian conflict or draw retaliation from Russia.
Planners took the view that the limited US missile strike on Shayrat airbase a year ago had failed to dissuade the Assad regime from using poison gas.
Before the green light was given by the prime minister for RAF participation in today’s attack, the defence secretary spent time with the attorney general going over the legality of the targeting in precise detail.
I am told that the lessons of the Chilcot Report – into the mistakes made over the Iraq invasion of 2003 – have been uppermost in people’s minds. Decisions, phone calls and sign-offs made over the last few days may one day have to be scrutinised by lawyers.
Russia too has been a major factor in choosing which targets to attack. Planners believe that the best way to mitigate against any Russian retaliation was to warn them in advance and avoid hitting any Russian positions in Syria.
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.
Addressing a press conference on Saturday, Mrs May said she believed the strike action had been “the right thing to do” and was “absolutely in Britain’s national interest”.
“This is not about intervening in a civil war. It is not about regime change,” she said.
“It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties.”
She said the UK government believed it was “clear” the Assad regime was responsible for the “despicable and barbaric” attack on civilians on 7 April.
Evidence suggested a barrel bomb and a regime helicopter had been used, she added. “No other group could have carried out this attack.”
Reporting from RAF Akrotiri in Cyprus, the BBC’s Jonathan Beale said the Tornados had left the airbase in the early hours of Saturday.
He added the cruise missiles had been fired “well away from Syrian airspace” and were out of the range of the regime’s air defences.