Sir Keir Starmer lost eight front MPs after suffering a major rebellion in a Commons vote calling for a ceasefire in Gaza.
Four shadow ministers, including Jess Phillips, Yasmin Qureshi, Afzal Khan and Paula Barker, resigned on Wednesday afternoon after deciding to support an SNP amendment to the King's Speech supporting the ceasefire.
Other frontbenchers Rachel Hopkins, Sarah Owen, Naz Shah and Andy Slaughter have also quit the frontbench after cracking the party whip to support the amendment.
Parliamentary private secretaries Dan Carden and Mary Foy have also left their posts.
MPs voted by 293 to 125, a majority of 168, to reject an amendment to the SNP's King's Speech calling for “all parties to agree to an immediate ceasefire” in Gaza.
But 56 Labor MPs backed the position, rejecting their party leader's stance.
Labor MPs had been ordered to stay away from the SNP's move and instead told to support Sir Keir's position calling for longer “humanitarian pauses” instead of a ceasefire.
In a statement after the vote, Sir Keir said he regretted that his party colleagues did not support his position.
“Along with leaders around the world, I have called for international law to be respected, for humanitarian pauses to allow access to aid, food, water, utilities and medicine, and have expressed our concerns about the scale of the loss civilians.
“Much more needs to be done in this regard to alleviate the humanitarian crisis unfolding in Gaza.
“And in addition to dealing with the present, every leader has a duty not to return to a failed strategy of containment and neglect, but to forge a better and more secure future for both Palestinians and Israelis.
“I am sorry that some colleagues felt unable to support the position tonight. But I wanted to be clear about where I was and where I would stand.”
Ms Phillips, a high-profile frontbencher, said it was with a “heavy heart” that she resigned.
“I tried to do everything I could to ensure that this was not the outcome, but it is with a heavy heart that I am stepping down from my position in the Home Office shadow team.
“On this occasion I must vote with my constituents, my head and my heart which has felt like it has been broken for the last four weeks by the horror of the situation in Israel and Palestine,” she said in a letter to her party. leader.
Some of the MPs had signaled their intention to break with Sir Keir in the Commons debate ahead of the vote, having already publicly called for a ceasefire.
Ms Shah warned of a “humanitarian disaster”, while Mr Khan told the room that “constituents have demanded” a ceasefire.
The scale of the rebellion will be a blow to Sir Keir, who had hoped to avoid further damaging splits within his parliamentary party over the issue.
The party has been divided by internal disputes over Israel's response to the deadly Hamas invasion that sparked the conflict.
The leadership backed the UK government's position to push for humanitarian pauses in the fighting to allow aid to Palestinians trapped in the bombed-out territories, but stopped short of calling for a complete cessation of hostilities.
Several of the frontbenchers made it clear why they believed Sir Keir's stance was wrong.
Ms Qureshi, who represents Bolton South East, said: “The situation in Gaza desperately requires an immediate ceasefire to address the humanitarian catastrophe and to promote moves towards a political solution that brings freedom, prosperity and security.” .
Some in Labor had accused the SNP of deliberately using the amendment to exploit divisions in the party.
But SNP Westminster leader Stephen Flynn said his MPs “will be able to look at themselves in the mirror knowing they did the right thing”.
“It is clear that support for a ceasefire would have been even stronger tonight if Keir Starmer had not threatened Labor MPs with punishment if they voted for peace,” he said.