Theresa May has said it was “legally and morally right” for the UK to join air strikes against the Syrian regime to prevent “further human suffering”.
She told MPs there was “clear evidence” the Assad government was behind the Douma chemical weapons attack.
The UK had “explored every diplomatic channel” in response but regrettably decided there was no alternative to “limited, carefully targeted action”.
But Labour leader Jeremy Corbyn said the action was “legally questionable”.
Calling on Mrs May to publish the legal advice in full, Corbyn said the government must be “accountable to this Parliament and not to the whims of this US President”.
He questioned who was responsible for the attack, saying that while it was “highly likely” the Assad regime was behind it, other groups had carried out similar attacks and weapons inspectors must continue their work.
Sites near Damascus and Homs were hit on Saturday by the US, French and UK in response to the alleged chemical attack on Douma on 7 April.
Both Syria, which denies any chemical use, and Russia, which provides military support to the Syrian government, have reacted angrily to the action.
UK prime ministers do not legally need to consult Parliament before launching military action, although they have done so since the 2003 invasion of Iraq.
Briefing Parliament, Mrs May insisted it was in the UK’s national interest to act – insisting that “we have not done this because President Trump asked us to but because it was the right thing to do”.
She said the use of chemical weapons could not be “normalised” in either Syria or elsewhere, insisting the attack was a “stain on humanity” and fitted the pattern of the regime’s previous use of such weapons.
“No other group could have carried out this attack,” she told MPs, adding that the Syrian authorities had reportedly attempted to “conceal the facts… supported by the Russians”.
The military response, she insisted, was aimed squarely at degrading the regime’s capacity to carry out further “indiscriminate” attacks, rather than to try and topple the regime.
Addressing calls for Parliament to have been given a vote, she said much of the intelligence on which the action was based could not have been shared with MPs and a speedy response was needed to protect the integrity of the operation and prevent further suffering.
“We have always been clear that the government has the right to act quickly in the national interest,” she said.
“I am absolutely clear that it is Parliament’s responsibility to hold me to account for such decisions but it is my responsibility as prime minister to make these decisions.”
Returning to the United Nations to try and secure a Security Council resolution would have been futile, she said, and effectively given Russia a “veto over British foreign policy”.
Corbyn called for a new War Powers Act, saying the convention that Parliament should be consulted before military action was “broken” and had to be replaced by a “legal obligation” to get the backing of MPs.
Contrary to what Mrs May claimed, he said it was evident all “non-military means had not been exhausted”.
“There is no more serious issue then the life and death matters of military action. It is right that Parliament has the power to support or stop the government from taking planned military action.”
Commons Speaker John Bercow has indicated he is likely to reject the PM’s request for a subsequent emergency debate in favour of one asked for by Labour MP Alison McGovern.
Any vote in these debates would be largely symbolic and would mainly just acknowledge that Parliament has had its say rather than give MPs the chance to formally approve or reject the air strikes themselves.
Legal advice from Dapo Akande, International Law professor at Oxford University, commissioned by Labour’s deputy leader Tom Watson, suggests neither the UN charter nor international law supports using humanitarian protection to support military action.
The advice says the government’s action risks undermining the UN Charter and is likely to be used by Labour to criticise the government’s case for strikes in the debate later.
The SNP’s Westminster leader Ian Blackford said for Parliament to be consulted about military action “after the event” was simply not good enough.
“When we are talking about military action in a third country, that is a decision that Parliament should be taking and not government itself,” he told BBC News. “That is what democracy is all about.”
Liberal Democrat deputy leader Jo Swinson said the decision had been taken “behind closed doors” and much more scrutiny was needed of the objectives behind it.
While the situation in Syria was “very serious”, she said there was not the same “acute urgency” for military intervention on humanitarian grounds as there had been in Libya in 2011.