The government's proposed £26,000 cap on benefits, one of the coalition's most divisive policies since coming to power, is facing a challenge from Liberal Democrats and Bishops in the Lords.
The bill will mean no family can claim more than £500-a-week in welfare, the equivalent amount of a £35,000 salary after tax.
As ministers face a showdown at the House of Lords, we take a look at who is for and against, and why.
FOR: Iain Duncan Smith, and the government. The work and pensions secretary insists the changes won't push families into poverty - and his policy is supported by 76% of the public according to YouGov polling.
IDS was backed up by the prime minister's official spokesperson who said on Monday "The key incentive we are trying to change is the incentive around work.
"We can't have a system where people are claiming benefits which are greater than you would get if you were in a job paying £35,000."
SITTING ON THE FENCE: Labour have taken a mixed position saying they support the camp in principle but are concerned about how it is being implemented. Shadow work and pensions secretary Liam Byrne has said Labour will support the bishops' amendment to exclude child benefit from the cap if the party's own amendment is defeated.
AGAINST: So why is it so controversial? In a blog for the Huffington Post CentreForum's chief economist Tim Leunig explains how an average family would have "only £1.64 to live on" under the cap.
"No alternative figures will make any difference: this is simply not a living income for a family with four children in private rented accommodation in a cheap part of outer London."
The Bishop of Ripon and Leeds, Rev John Packer, is attempting to change the bill to mean child benefit is excluded. He is supported by the child poverty action group, who argue that the policy is "based on a foundation of myths" and could push 210,000 children into "severe poverty and homelessness.
Chief Executive Alison Garnham added the Bishops' changes were sensible. "We hope a government that promised to be the most family friendly ever will prove it today by following their lead."
Take a look at our slideshow for a run-down of who's for and who's against the cap.
"I voted with the Government on everything until now [but] I will not support the benefit cap in its present form... "The work and pensions Secretary Duncan Smith has said that he accepts the transition arrangements have to be put in place. I know, I know, that Nick Clegg is fighting very, very hard for those transition arrangements to be put in place." The former Liberal Democrat leader added: "As the President of UNICEF, I think the effect on children across the country of a cap in its present form will be in my view completely unacceptable."
Research carried out by Shelter and the Chartered Institute of Housing in March 2011 showed that the proposed reforms will have a severe impact on families' ability to find decent, affordable homes to rent.
"Firstly we are concerned that the savings from this measure, currently estimated ay £270m savings p.a from 2014-2015 does not take account of the additional costs to local authorities (through homelessness and temporary accommodation). In fact we think it is likely that the policy as it stands will generate a net cost," the communities secretary allegedly said in a letter leaked to the Observer.
"The government needs to do further work to prevent harmful consequences of the benefits cap if the changes are to be accepted by Parliament," the Liberal Democrat deputy leader has said. "As it currently stands the benefits cap will break up families, as it will provide a financial incentive to be apart."
The Bishop of Ripon and Leeds, the Rt Rev John Packer has tabled an amendment which would exclude child benefit payments from the £500-a-week limit. "Child benefit is a universal benefit. I believe that it's wrong to see it as being a welfare benefit. It's a benefit which is there for all children, for the bringing up of all children and to say that the only people who cannot have child benefit are those whose welfare benefits have been capped seems to me to be a quite extraordinary argument."
"The household benefit cap policy is built on a foundation of myths, but the 210,000 children affected will face harsh realities of severe poverty and homelessness. "The Bishops in the House of Lords will be putting forward some sensible proposals today that will protect children and families. We hope a government that promised to be the most family friendly ever will prove it today by following their lead," the Chief Executive of Child Poverty Action Group, Alison Garnham, said.
Nick Clegg is behind the government despite the division in his party. The deputy prime minister has suggested there was some scope for softening the impact of the changes through "transitional arrangements" around the introduction of the cap.
"The reality here is that £26,000 a year, there is absolutely no reason why any family should not be able to be found accommodation and no children should be in any respect plunged into poverty," he told BBC Radio 4's Today programme.
A YouGov poll has shown over three quarters of the country, 76%, back the welfare cap.
"Labour won't be voting against the benefits cap because we support the principles and the responsibility to take a job if you can work. "But we will be seeking to amend the bill, to bring a compromise between the bishops and the government because we don't think council tax payers should be hit with a massive bill for homelessness. In tough times, we don't want this idea to end up costing more than it saves", a Labour spokesperson said