"Yet... the worldwide community is not convinced that a chemical attack took place in Douma, and wants to see the results of the ongoing OPCW investigation... to say that "there is no practicable alternative to the use of force if lives are to be saved" is hypocritical, to say the least", the statement says.
"The prime minister has the full authority, on the basis of all the information at her disposal, to order the type of military action which has been carried out this morning and we reject any suggestion that she was not entitled to do so".
She declined to say whether Bashar al-Assad should stay in power and said talks with allies would continue on finding a political solution to the civil war.
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.
"Given the context of the recent worldwide response to the use of a nerve agent in the United Kingdom, the clear targeted objective of the strikes, and the repeated blocking by Russian Federation of diplomatic solutions through the UN, we believe the Prime Minister was justified in standing with our American and French allies in this concerted action".
May has repeatedly said that the missile strike on Syria was not about "regime change".
Russia, which intervened in the war in 2015 to back Assad, has denied there was a chemical attack and has accused Britain of helping to stage the Douma incident to stoke anti-Russian hysteria.
On Friday night the US, Britain and France launched air strikes against Syrian chemical sites after obtaining "proof" that poisonous gas was used last weekend in Douma, killing 41 civilians.
Correspondents from the British and world press also asked the prime minister to explain what role can the Parliament play in making a decision to attack another sovereign country.
Parliament is not due to reconvene until Monday, following its Easter recess.
May is not obliged to win parliament's approval before ordering military action, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the USA -led invasion of Iraq.
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.
Scottish First Minister Nicola Sturgeon also said the strikes risked "dangerous escalation".