Labour will force a Commons vote on proposals for a £1.3 billion-a-year cut in disability benefits as ministers faced pressure from Tory backbenchers to perform a U-turn on the plans.
The Government could face a humiliating defeat if concerned Tories join with Jeremy Corbyn's party in blocking what the Labour leader called the "appalling" plan to impose tighter restrictions on the personal independence payment (PIP).
Mr Corbyn said 200,000 of the 640,000 hit by the changes would lose out altogether as a result of the Government's plan to "arbitrarily" make the cuts.
In an effort to prevent a revolt which could wipe out the Government's slender Commons majority, Work and Pensions Secretary Iain Duncan Smith wrote to Tory MPs to insist that consultations were continuing, while Cabinet colleague Nicky Morgan said the proposed cut was merely a "suggestion".
But Mr Corbyn told BBC Radio 5 Live the Government was planning to take £4 billion out of the benefit over the course of the Parliament at the same time that George Osborne's Budget was "giving tax relief to the biggest companies in cutting corporation tax".
"It is utterly appalling what they are proposing," he said. "We will be forcing a vote in Parliament on this. I hope and believe all opposition parties will join with us in that.
"I believe a number of Conservative MPs are so upset about this they too will vote against the Government."
Conservative backbencher Andrew Percy, who has organised a letter to the Chancellor calling for a rethink, warned that the Government would be defeated in the Commons if it tried to push the changes through.
Backbencher William Wragg became the latest Conservative to register his concern about the plan, putting his name to Mr Percy's letter and saying the "PIP changes need a rethink".