Tory divisions over a controversial benefit reform could be exposed with up to 25 Conservative MPs tipped to back a Labour vote in the House of Commons.
Labour will hope to win a symbolic victory in an opposition day vote which will call for the roll-out of Universal Credit reforms it says are making "millions worse off" to be paused.
Jeremy Corbyn and Theresa May clashed over the reforms at Prime Minister's Questions ahead of the evening vote.
Mr Corbyn's party has already won a victory after the government scrapped Universal Credit helpline charges - that could cost up to 55p a minute - a week after he challenged Mrs May at PMQs.
What is the Universal Credit row?
Universal Credit combines a number of benefits such as housing benefit and tax credits into a single payment.
It has been dogged by criticism that claimants are waiting six weeks for the money and getting into debt.
From October the pace of its roll-out was set to be ramped up, with 50 Jobcentres moving to the service every month.
Government figures showed 23% of new claimants do not receive their first full payment within six weeks, which has been linked to rent arrears and other debts for claimants.
What is Labour calling for?
Labour have called on the government to "pause and fix" the benefit.
Welcoming the scrapping of the helpline charges at PMQs, Mr Corbyn said: "The fundamental problems of Universal Credit remain. The six-week wait, rising indebtedness, rent arrears and evictions."
Work and Pensions Secretary David Gauke has been accused of overseeing a department that has "no idea" about the operation of the policy by Work and Pensions Select Committee chairman Frank Field.
The Tory minister was grilled by MPs on the committee, during which he confirmed the helpline charges were being scrapped.
He insisted the roll-out will continue, with the system tweaked so claimants are offered advance payments upfront.
What has Theresa May done to sway the rebels?
The prime minister has held talks with would-be rebels in an effort to stave off an embarrassing revolt.
However Mrs May appeared reluctant to concede ground as she defended the "simpler" benefits system in the Commons during PMQs.
"It is a system that encourages people to get into the workplace," she said. "It is a system that is working because more people are getting into work."
Heidi Allen, Sarah Wollaston and Johnny Mercer are among the MPs that Mrs May has spoken to ahead of the non-binding vote.
Tory MP Douglas Ross will notably not be among the rebels though - he is unable to attend the vote as he is running the line at a Champions League match in Barcelona.
If Labour's vote is non-binding what affect can it have?
Quite a bit if it shows the ruling party is in the minority.
Last month, Labour received backing from the Democratic Unionist Party, which is propping up Mrs May's minority Government, in opposition day votes on NHS pay and tuition fee increases.
That forced the Tories to abstain on the non-binding motions.
Shadow work and pensions secretary Debbie Abrahams has urged those on the opposite benches to "vote with their conscience" and rebel.
She said: "The Government has so far not listened to MPs' concerns about the mounting issues with their flagship social security programme.
"The social security system is meant to protect people from debt and arrears, not exacerbate their situation.
"We must pause and fix Universal Credit now, before millions are made worse off."