Deputy Prime Minister Nick Clegg has defended the Government's plan for a real-terms cut in most working-age benefits, even though most of those affected will be in employment.
Speaking ahead of a crunch Commons vote on the plan, which will see rises limited to a below-inflation 1%, Mr Clegg said it would save £5 billion over the next three years.
But deputy Labour leader Harriet Harman said the Government had "failed on compassion" and condemned the decision to impose the cut at the same time as reducing the top rate of income tax.
Liberal Democrat former minister Sarah Teather and John Leech have vowed to rebel and vote with Labour against the Benefits Up-rating Bill's second reading. Ms Harman said Mr Clegg and loyal Lib Dems would be "voting to make millions of low-income families worse off" and two-thirds of those hit were in work.
Mr Clegg said: "It's obvious that a measure which deals with both out-of-work benefits and tax credits affects people both in and out of work."
But he said Labour had to explain why it could support a 1% limit on pay rises for "doctors, teachers, nurses in the public sector" and "not take exactly the same approach in this area".
At Mr Clegg's regular question time appearance in the Commons, Ms Harman said: "One per cent if you are earning over £100,000 a year is a great deal more than 1% if you are struggling on a low income."
The Government was "failing on the economy" and borrowing £212 billion more than planned.
She asked Mr Clegg: "When it comes to fairness, will you admit that the vote will mean that, while someone earning over £1 million a year will be better off by £2,000 a week because of their tax cut, a working couple on tax credit will be worse off as their increase of 38p a week will be wiped out by inflation?"
Mr Clegg said the decision to raise the income tax threshold would benefit more than 20 million basic-rate taxpayers.