What is the benefit cap?
A cornerstone of Iain Duncan Smith's welfare reform, the principle behind the benefit cap is that unemployed people should not be paid more than working families. Under the proposals, working-age benefits would be capped at £500 per week, or £26,000 a year. This is equivalent to the average wage earned by working households after tax.
What do voters think?
The public are broadly with Duncan Smith on this one. A YouGov poll shows that 76 per cent of the public are in favour of the benefits cap, including 69 per cent of Labour voters. It also shows that 36 per cent would like to see even tougher measures, with no household getting more than £20,000 in welfare payments.
Why is it unfair?
There are several criticisms levelled against the cap, centering on the impact on children, and on the very families -- those in work and paying taxes -- who it is meant to defend.
It takes no account of children
The benefit cap is the same regardless of how many children a family has. Therefore a family with five children would receive the same amount as a family with just one or two. This has proved particularly problematic, with church leaders have called for child benefit to be exempted, while Nick Clegg conceded that government may need to look at "the place of children who were born, if you like, innocently into another set of rules".
Leaked government analysis  showed that the move could push a further 100,000 children into poverty. Duncan Smith's team are scathing about an amendment that would exempt child benefit, saying it would encourage people on benefits to have more children. But what about families that already have many children? They should not be penalised for the fact of their existence.
It ignores employment history
A couple who have never worked will end up being less affected than families on a low to middle income who are suddenly affected by unemployment in the recession. This is because families with parents who have never worked will tend to live in social housing where rents are cheaper. However, low to middle earners are likely to rent privately: they are not poor enough to qualify for council accommodation but not well off enough to buy. There are 680,000 working households claiming housing benefit, making up 14 per cent of the total housing benefit caseload.
As the recession claims more and more jobs, many families who need short-term support could find themselves in an impossible position. Duncan Smith has criticised "people being placed in houses they cannot afford", but for these families, it is a case of rapidly changing circumstances rather than flagrantly living above their means.
It penalises those in the south-east
Rents are higher in the south-east, and cutting housing benefit to £100 a week makes it practically impossible for a family with children to rent privately. In the Guardian today, Tim Leunig  says that after council tax, rent and utilities, a family with four children would be left with 62p per person per day to live on.
Critics have said that this will result in "social cleansing" from inner-city areas -- the percentage of privately rented properties in central London available to housing benefit claimants will fall from more than 50 per cent to just 7 per cent. Leunig predicts that many people will remain in these areas, where job prospects are better, but will have to "downsize", with siblings sharing bedrooms and parents sleeping on sofa-beds.
Who opposes it?
Lord Paddy Ashdown, the former Liberal Democrat leader, has said  that he is unable to support the reform as it will unfairly penalise children in benefit dependent homes. He said:
I voted with the Government on everything until now. I see it as my job as an ex-leader to support my successor, but I will not support the benefit cap in its present form.
Church of England bishops, led by John Packer, the Bishop of Ripon and Leeds, will today table the key amendment around which rebel Lords plan to gather. It would exempt child benefit from the cap. As outlined above, the government is hostile to such a plan, though Clegg has suggested that there may be some scope for "transitional arrangements" to cushion the effect.