Speaking at the House of Commons on Monday, Theresa May said the UK’s involvement in Saturday’s attacks on Syria’s alleged chemical weapons sites was in the national interest.
“We have not done this because [US] President [Donald] Trump asked us to do so,” she told parliament.
“We have done it because we believe it was the right thing to do, and we are not alone.”
Many opposition MPs have questioned May’s decision to join the attack without seeking parliamentary approval.
Jeremy Corbyn, leader of the opposition Labour Party, reiterated his stance that May is “accountable to the parliament – not to the whims of the US president”.
He also questioned the legal justification for the attack, saying it was not in line with the UN Security Council’s charter.
“There is no more serious issue than the life and death matters of military action,” he said on Monday.
“It is right that parliament has the power to support or stop the government from taking planned military action.”
For its part, the Scottish National Party says it will force a vote on the issue.
Since the Iraq war in 2003, under Britain’s constitutional convention parliament expects an opportunity to debate the matter if British troops are to be involved in military combat.
Confirming UK involvement in the attack in her speech, May said: “We would have preferred an alternative path. But on this occasion there is none.”
She said the attacks were not about “regime change” or “intervening in a civil war”, but were to “deter the use of chemical weapons” by the Syrian government.
After her statement, May was expected to ask the speaker of parliament to grant an emergency debate, followed possibly by a non-binding vote.
Al Jazeera’s Jonah Hill, reporting from London, said: “I don’t think Monday’s session will satisfy the opposition parties, who are determined that they should have had, and could still retrospectively have, a definitive vote on the issue of the strikes themselves.”