Around 108 Labour backbenchers have signed an amendment to stop the bill, with some ministers also reportedly considering rebellion.
The prime minister insists the vote is not a confidence matter, but internal party sources suggest a potential defeat could threaten the position of chancellor Rachel Reeves.
Starmer argues the reforms are essential to fix a 'broken' and unsustainable welfare system, while trade unions warn against relying on Tory votes to pass the cuts.