Starmer suspended the Labour rebels late on Tuesday after they backed a motion demanding the removal of the contentious two-child limit on benefits introduced by the previous Conservative government.
Their votes supporting ending the cap – introduced in 2015 and which restricts payments to the first two children born to most families – is an early test of Starmer’s authority.
The new UK leader has warned there is “no silver bullet” to ending child poverty but acknowledged the “passion” of MPs who oppose maintaining the policy.
Starmer’s decision to suspend the whip from the group of left-wingers, which included former finance spokesman John McDonnell, was seen as a show of ruthlessness from his new administration.
The Labour leader took power just weeks ago after his party, in opposition for 14 years from 2010, won a landslide in the July 4 general election.
The victory followed a four-year struggle since he became party leader to shift Labour back to the political centre ground from the hard-left regime of former leader Jeremy Corbyn.
The party in 2019 experienced its worst election result in nearly a century under Corbyn.
Starmer will be on his feet in the Commons at 12pm local time for his first weekly Prime Minister’s Questions session, when the politically charged two-child cap could feature.
Late on Tuesday, MPs voted 363 to 103 to reject a Scottish National Party (SNP) amendment to scrap the cap, giving the government a majority of 260.
However, in addition to the seven who voted with the amendment, more than 40 Labour lawmakers recorded no vote, highlighting the level of unease within the centre-left party at the measure.
Liverpool MP Kim Johnson said she had voted with the government “for unity” but warned that the strength of feeling within the party was “undeniable”.
“We moved the dial, the campaign will continue,” she said.
The SNP’s Westminster leader Stephen Flynn said Labour had “failed its first major test in government” by choosing not to “deliver meaningful change from years of Tory misrule”.
“This is now the Labour government’s two-child cap – and it must take ownership of the damage it is causing, including the appalling levels of poverty in the UK,” he said.