"This legally questionable action risks escalating further", Corbyn, a veteran anti-war campaigner, said.
Top Photo | British Labour Party leader Jeremy Corbyn speaks to launch his bid to retain the leadership of the party, at the University College London Institute of Education in London, Thursday, July 21, 2016.
"May should have sought parliamentary approval, not trailed after Donald Trump", he said on Saturday.
Parliament should have been consulted and voted on the matter.
With U.S. President Donald Trump reportedly mulling his "options" for a military attack on Syria while continuing to issue belligerent threats on Twitter, U.K. Labour Leader Jeremy Corbyn condemned the Tory government on Friday for "waiting for instructions" from Trump and demanded an immediate resumption of peace talks.
Mr Corbyn said the alleged chemical attack in Douma was "disgraceful", but added: "There has to be a political solution".
There have been calls from opposition parties and some Conservative MPs for Parliament to have a vote beforehand.
Corbyn is right, bombs will not save lives.
Mr Cable said his party had been willing to back a "properly planned and justified" intervention in Syria, but he warned today that the decision to move ahead without Parliament "fatally undermines the integrity of this mission". 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.
Russia's military said on Thursday that Douma was now under full control of the Syrian government after a Russian-mediated deal secured the evacuation of the rebels and thousands of civilians after it was recaptured by Syrian forces.
She was asked three time whether she believed Russia or the United States was a greater threat to world peace, before conceding that, "at this point", given its role in Syria and Salisbury, Russia posed the higher risk.
During Thursday's cabinet meeting called to discuss the United Kingdom response, she described it as "shocking and barbaric" and said it was a "further example of the erosion of global law in relation to the use of chemical weapons, which was deeply concerning to us all".
The strikes took place only hours before an investigation by inspectors from the UN's Organisation for the Prohibition of Chemical Weapons (OPCW) was due to start.
Mrs May and Mr Trump had also "agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime".
That view was echoed by Liberal Democrat leader Vince Cable, who insisted that Ms May should have sought the approval of MPs before following the USA into Syria.