Tom Watson, the deputy Labour leader, released the five-page legal opinion from Dapo Akande, a professor of public international law at Oxford University.
A summary of Akande’s conclusions were published on Monday by the Guardian newspaper.
“Contrary to the position of the [UK] government, neither the UN charter nor customary international law permits military action on the basis of the doctrine of humanitarian intervention,” the opinion said.
“The legal position advanced by the government ignores the structure of the international law rules relating to the use of force,” it added.
“The action taken by the government was not directed at bringing “immediate and urgent relief” with regard to the specific evil it sought to prevent, and was taken before the inspectors from the Organization for the Prohibition of Chemical Weapons were able to reach the affected area.”
Meanwhile, Labour leader Jeremy Corbyn has described airstrikes on Syria as legally questionable and accused UK Prime Minister Theresa May of “trailing after” US President Donald Trump in an attack that could escalate the conflict.
Corbyn, a veteran anti-war campaigner, said Saturday that May should have sought approval from the UK Parliament before ordering the attack.
“Bombs won’t save lives or bring about peace,” Corbyn said. “This legally questionable action risks escalating further.”
“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,” he added.
Labour has opposed a military strike on Syria since the suspected chemical weapons attack on Douma near the cap[ital Damascus.
American, British and French forces launched air strikes on Syria early Saturday in response to a suspected chemical weapons attack.
Syria has strongly rejected any role in the suspected attack, which took place just as the Syrian army was about to declare full victory against the militants operating in the Eastern Ghouta region near Damascus.
Syria, Russia and Iran say reports of the attack were fabricated by militant groups and rescue workers and have accused the United States of seeking to use it as a pretext to attack the Syrian government.