British jets fired missiles at a Syrian military base suspected of holding chemical weapons ingredients on Saturday in Britain's first military action against President Bashar al-Assad's regime.
Prime Minister Theresa May said the "limited and targeted strike" was part of joint action with France and the United States in response to Syria's latest alleged chemical weapons atrocity.
"There is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime," May said in a televised statement.
"This is not about intervening in a civil war. It is not about regime change.
"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.
Despite her careful language, there was immediate criticism from opposition lawmakers who said May should have consulted parliament before joining US-led action in Syria.
But May said intelligence pointed to the Syrian government being behind the suspected chemical attack on the rebel-held Damascus suburb of Douma last Saturday, and Defence Secretary Gavin Williamson said speed was "essential".
May said the strikes would "send a clear signal to anyone else who believes they can use chemical weapons with impunity".
"This is the first time as prime minister that I have had to take the decision to commit our armed forces in combat –- and it is not a decision I have taken lightly.
"I have done so because I judge this action to be in Britain's national interest," she added.
In her comments, May also alluded to a nerve agent attack in Britain last month on a former Russian spy and his daughter.
"We cannot allow the use of chemical weapons to become normalised -- within Syria, on the streets of the UK, or anywhere else in our world," she said.
Britain has blamed Russia for the poisoning -- a charge vehemently denied by Moscow which has accused London of failing to come up with evidence for its claims.
Britain's defence ministry said in a statement that four British Tornado jets had fired Storm Shadow missiles at the Syrian base 15 miles (24 kilometres) west of Homs at 0100 GMT.
"Very careful scientific analysis was applied to determine where best to target the Storm Shadows to maximise the destruction of the stockpiled chemicals and to minimise any risks of contamination to the surrounding area," the ministry said.
Williamson told BBC radio that all British crews returned safely and early indications were that the strikes had been "highly successful".
"Our service personnel have played an important role in terms of degrading the ability of the Syrian regime to use chemical weapons in the future," Williamson said.
May held an emergency cabinet meeting to discuss possible action on Thursday and there had been calls for the British parliament to be consulted before any air strikes.
Parliament is not due to reconvene until Monday, following its Easter recess.
Polls in recent days have shown public wariness of military intervention in Syria, with Britain still haunted by its participation in the US-led invasion of Iraq.
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
But they backed action in Iraq the following year, and again in Syria in 2015, strictly limiting strikes to targets of the Islamic State group.
"Bombs won't save lives or bring about peace," Jeremy Corbyn, leader of the main opposition Labour Party, said in a statement.
"This legally questionable action risks escalating further... an already devastating conflict," he said, adding that May should have sought parliamentary approval.
Other opposition leaders joined in the criticism.
Scottish First Minister Nicola Sturgeon also said the strikes risked "dangerous escalation".
Vince Cable, leader of the Liberal Democrats, said: "Riding the coattails of an erratic US president is no substitute for a mandate from the House of Commons".