Air strikes on Syria were legally justified on humanitarian grounds, government documents say.
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”, tweeting: “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”.
The Security Council rejected a resolution drafted by Russia, which condemned “the aggression against the Syrian Arab Republic by the US and its allies”. A resolution needs nine votes to pass and only Russia, China and Bolivia voted in favour.
Meanwhile, Nato secretary general Jens Stoltenberg said all allies had given the action their full support.
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.
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”
By Frank Gardner, BBC security correspondent
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.
PM faces ‘almighty row’ over Syria strikes
By Laura Kuenssberg, BBC political editor
Prime ministers don’t choose the decisions that face them. But they have to judge which way to jump.
In 2013, Theresa May’s predecessor tried and failed to get approval for military action against President Assad. There was international alarm, then as now, about his suspected use of chemical weapons.
But MPs rejected David Cameron’s plan and he didn’t try again to persuade Parliament it was necessary. This time, she has avoided that particular obstacle by taking action alongside the US and France while MPs are away.
In the UK, party leaders including Scotland’s First Minister Nicola Sturgeon and the Liberal Democrat’s Sir Vince Cable said MPs should have had a vote on the action – while Plaid Cymru’s Leanne Wood accused the government of putting UK security at risk.
But Northern Ireland’s Democratic Unionist Party said Mrs May had “the full authority” to order the air strikes.
What did the UK use?
The RAF sent four Tornado GR4 aircraft from Cyprus, each armed with two Storm Shadow cruise missiles.
The Tornado has been one of the mainstays of the RAF since first entering service in 1980 and the aircraft were used to enforce no-fly zones in Iraq.
Weighing in at 2,866lb (1,300kg), measuring 16.7ft (5.1m) in length and with a range in excess of 150 miles (240km), the “bunker busting” Storm Shadow has been described by the air force as “arguably the most advanced weapon of its kind in the world”.
The range means that none of the GR4s would have been required to cross into Syrian airspace to launch the assault.
The aircraft were sent from RAF Akrotiri, a British base since the 1950s on the southern side of Cyprus, some 213 miles (343km) west of Homs.