Jeremy Corbyn said the military action against Syria was “legally questionable” and makes real accountability for war crimes less likely.
The Labour leader was speaking after four RAF jets joined French and American forces in attacking the Assad regime over the alleged chemical attack on civilians.
Prime Minister Theresa May said there was “no practicable alternative to the use of force” to deter the use of chemical weapons and Defence Secretary Gavin Williamson said the situation was too urgent to recall parliament for a vote.
Corbyn said: “Bombs won’t save lives or bring about peace.
“This legally questionable action risks escalating further, as US defence secretary James Mattis has admitted, an already devastating conflict and therefore makes real accountability for war crimes and use of chemical weapons less, not more likely.
“Britain should be playing a leadership role to bring about a ceasefire in the conflict, not taking instructions from Washington and putting British military personnel in harm’s way.
Corbyn continued: “Theresa May should have sought parliamentary approval, not trailed after Donald Trump.
The Government should do whatever possible to push Russia and the United States to agree to an independent UN-led investigation of last weekend’s horrific chemical weapons attack so that those responsible can be held to account.”
May is certain to face MPs’ anger on Monday when parliament returns from recess. In 2013, MPs voted down British military action against the Assad regime and the latest incident raised questions over whether they should be allowed another vote.
Lib Dem leader Sir Vince Cable said the failure to do this “fatally undermines the integrity of this mission”.
“It shows a weak Government putting short term political expediency before democracy and in so doing further diminishing the standing of Britain in the world,” he added.
“Riding the coattails of an erratic US President is no substitute for a mandate from the House of Commons.
“The Prime Minister could and should have recalled Parliament this week and sought the approval of MPs before proceeding.”
The SNP is pushing for an emergency debate on the matter.
Nicola Sturgeon said: “There has been no explanation from the Prime Minister or the President on how this action, taken without parliament’s approval, will halt the use of chemical weapons or contribute to a long term peace in the area.
“Air strikes by US and UK forces have not resolved the situation in Syria in the past and I am not persuaded they will do so now.
“This action risks not just further escalating the civil war in Syria but also a dangerous escalation of international tensions.”
Speaking at Downing Street on Saturday morning, May said the strikes were “limited and targeted” and designed to degrade the Syrian regime’s ability to develop and use chemical weapons.
She told reporters: “The action that took place last night was an action which was focused on degrading and deterring the operational capability and the willingness of the Syrian regime to continue to use chemical weapons.
“There have been many instances when we have seen them using those chemical weapons.
“But I believe it should also be a message to others that the international community is not going to stand by and allow chemical weapons to be used with impunity.”
May said chemical weapons had “all too often” been used in recent times.
“I think it is right that the international community has come together and said we will not accept this,” the prime minister added.