LONDON—The U.K. government's push for Britain to join U.S.-led airstrikes against Islamic State in Syria has laid bare deep rifts within the main opposition Labour Party, posing a crucial test for the authority of its new leader, Jeremy Corbyn.

Mr. Corbyn has faced public dissent from members of his party, including some in his top team, over his opposition to military intervention. Following intense pressure from some within his party, Mr. Corbyn, a veteran peace campaigner, agreed to allow Labour lawmakers to follow their conscience in a parliamentary vote on a government motion designed to pave the way to extend British airstrikes to Syria from Iraq, where it has been bombing Islamic State for more than a year.

That move has improved British Prime Minister David Cameron's chances of securing a majority in the vote, which is likely to take place late Wednesday. Mr. Cameron has said he would only hold a vote if he was confident of winning.

In an unusual move, Mr. Corbyn will open the debate for Labour arguing against military intervention, while others, including his spokesman on foreign affairs, are expected to argue in favor.

The event marks the most high-profile test of Mr. Corbyn's leadership since he took the top job in September in the wake of the Labour Party's painful loss in national elections in May. While his antiausterity and antiwar message appeals to many grass-roots supporters, some members of the parliamentary party have expressed concern that his stance is too far left to win a general election.

The dissent on Syrian strikes follows a series of other internal spats over policies ranging from the economy to Europe that have put Mr. Corbyn at odds with other Labour lawmakers, some of whom fear his leftist views and close ties to unions will alienate voters.

Mr. Cameron has won over some Labour lawmakers, including foreign-affairs spokesman Hilary Benn, with his arguments about the need to intervene in Syria and the threat Islamic State poses to the Middle East and the U.K.

But Mr. Corbyn remains strongly opposed. He has argued that bombs will kill innocent civilians in Syria, that the Syrian forces the prime minister would be relying on are infiltrated by jihadists, and that no end point is in sight.

The Labour leader also said the vast majority of rank-and-file members oppose airstrikes and that he believes all efforts should go into finding a political solution to end the war.

At a meeting of Labour lawmakers on Monday, Mr. Corbyn failed to convince his top team to vote against the prime minister amid concerns some could quit if they were forced to do so. Mr. Corbyn then announced Labour lawmakers were free to vote as they wished on the issue, a move one of his top supporters had said would hand victory to the prime minister "on a plate."

"At first there were some people who were prepared to give him [Corbyn] the benefit of the doubt, but I think there are a lot of people now who think he is just not up to the job," said Tim Bale, a politics professor at Queen Mary University of London.

Labour has dealt with divisions over war before: in 2003 the then Labour Foreign secretary Robin Cook resigned as from Prime Minister Tony Blair's cabinet because he disagreed with military action in Iraq.

Write to Nicholas Winning at nick.winning@wsj.com

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(END) Dow Jones Newswires

December 01, 2015 19:45 ET (00:45 GMT)