Senior Tories are making a last-gasp bid to block the £20-a-week cut to universal credit, by staging a Commons showdown on Monday.
They have tabled an amendment to the annual uprating of pensions, which would block the increase unless funds are diverted to stop the benefit reduction.
The reduction – which will kick in next month – is predicted to plunge half million more people into poverty, including 200,000 children.
An internal Whitehall analysis warned of a “catastrophic” impact from removing the support, including rising homelessness, poverty and foodbank use.
Mr Johnson came under fire when he refused to explain how universal credit claimants should recoup the £20-a-week and criticised putting taxes “into benefits”.
The vote on Monday is crucial, to uprate pensions and other benefits next April, and is normally considered a formality with little drama.
The amendment tabled by Mr Duncan Smith and Mr Green would prevent that uprating going ahead if they can persuade more than about 40 fellow Tories back them – a formidable task.
One rebel told The Independent: “It cannot be a straight vote on the universal credit cut, but we want to force the government to put money into keeping the uplift.
“We expect there to be a large rebellion and it does at least give Conservative MPs the chance to have a say on this. The whips will be furious.”
Labour staged a Commons vote on the universal credit cut last Wednesday, but it was non-binding and Tory MPs were told to abstain, allowing the motion to pass.
Thérèse Coffey, the work and pensions secretary, sparked anger as she got her sums badly wrong by arguing claimants should find more work because “£20 a week is about two hours’ extra work”.
It was quickly pointed out that Universal Credit is deliberately “tapered”, so a huge chunk of the payment is taken back as earnings rise.
The respected Resolution Foundation think-tank said claimants take home as little as £2.24 per for every hour worked on the national minimum wage of £8.91, after travel and childcare costs.
They would need to work an extra six hours a week to make up the £20 cut in support – rising to nine hours if they pay tax and National Insurance, it found.
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