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".
He warned that intervention would lead to a proxy war with Russian Federation which would be "not only risky to Britain, but the entire world".
Theresa May said "everything possible" was done to deter civilian casualties.
"I want to state categorically ... that Britain has no involvement and would never have any involvement in the use of a chemical weapon", she said, the Guardian reported.
Britain's defence ministry said in a statement that four British Tornado jets had fired Storm Shadow missiles at the Syrian base 24 km west of Homs at 0100 GMT.
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.
"While the full assessment of the strike is ongoing, we are confident of its success", she added.
Shortly after the military strikes were launched, Scotland's first minister Nicola Sturgeon said United Kingdom foreign policy should be set by Parliament and not Donald Trump after the U.S., United Kingdom and France bombed targets in Syria. Western governments, including Britain, blamed the government of Bashar al-Assad, a Russian ally, for the attack.
The British PM said the attack was specifically a reaction to the use of chemical weapons, and was not about a regime change.
In Douma, last Saturday a chemical weapons attack killed up to 75 people, including young children, in circumstances of pure horror.
"I believe it should also be a message to others that the global community is not going to stand by and allow chemical weapons to be used with impunity." she asserted.
I have done so because I judge this action to be in Britain's national interest.
The Syrian Regime has a history of using chemical weapons against its own people in the most cruel and abhorrent way.
He also did not rule out future military action.
Prime Minister Theresa May during a press conference in 10 Downing Street, London on the air strikes against Syria.
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.
Mrs May said chemical weapons had "all too often" been used in recent times.
A North Atlantic Treaty Organisation meeting will be held on Saturday during which the UK, France and the U.S. will brief allies on the action taken in Syria.
Iranian President Rouhani says US-led military strikes in Syria will cause "destruction" in the Middle East.