Jeremy Corbyn has described air strikes in Syria by US, British and French forces as a "legally questionable action".
The Labour leader condemned the attack aimed at reducing the Syrian government's chemical weapons capabilities, claiming it would make real accountability for war crimes less likely.
Mr Corbyn said Prime Minister Theresa May should have sought parliamentary approval for Britain’s participation in the strikes, rather than "trailing" after US President Donald Trump.
"Bombs won't save lives or bring about peace," he said in a statement on Saturday.
"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.
"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."
Mrs May circumvented a non-binding constitutional convention dating back to the 2003 invasion of Iraq by launching strikes without prior approval from parliament.
However, the Prime Minister claimed speed was essential and the action against Syria was in Britain’s national interest.
Allied air strikes were carried out in Syria overnight in response to a suspected poison gas attack that killed dozens of people in Douma near Damascus last Saturday.
The intervention is the largest so far by Western powers in the Syrian conflict, with more than 100 missiles launched at various targets.
The Ministry of Defence says four Tornado jets from the RAF’s Akrotiri base in Cyprus fired missiles at a military facility near Homs where it was believed Syria had stockpiled chemicals.
Russia, an ally of the Syrian government led by president Bashar al-Assad, has warned the strikes will have "consequences".
Russian president Vladimir Putin has claimed the attack will exacerbate the humanitarian crisis in the country.
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