Tax credit cuts will cause unacceptable "suffering" for poorer families, Ruth Davidson has warned, as she joined a growing chorus of Tory demands for a rethink by George Osborne.
The highly-popular leader of the Scottish Conservatives said it was wrong that claimants faced a "cliff edge" when their state help was slashed in April - before any benefits are felt of a higher minimum wage.
Peers are threatening to block the £4.4 billion welfare squeeze in a vote on Monday, threatening a constitutional showdown with the Government if the upper house defies convention and fails to bow to the will of the Commons on financial measures.
Mr Osborne has refused to back down, insisting rising wages will offset the effects, but with senior Tory MPs leading a Commons rebellion later in the week, it was reported he could unveil mitigation measures in his Autumn Statement on November 25.
Ms Davidson said the cuts were necessary but needed to be introduced more carefully.
"It's not acceptable. The aim is sound, but we can't have people suffering on the way," she told the Mail on Sunday.
"The idea that there's a cliff edge in April before the uptake in wages comes in is a real practical human problem and the Government needs to look again at it."
Mr Osborne insisted this week he was "comfortable" with the policy despite analysis by the independent Institute for Fiscal Studies (IFS) that the introduction of a higher minimum wage will not offset poor households' immediate losses.
The IFS calculates that millions of people will lose up to £1,300 a year from the Budget changes.
Liberal Democrats in the House of Lords have tabled a "fatal" motion that would kill off the policy - with David Cameron not ruling out appointing hundreds of new Tory peers to give the Government a majority in retaliation.
Party leader Tim Farron said the Prime Minister could "stuff the convention" and declared he would "take great pleasure" in hoisting David Cameron by his own petard by using the unelected Lords to block government legislation after the Tories blocked reform of the upper house.
Labour instead proposes a move to delay a final decision until the Government sets out "full transitional protection for a minimum of three years" for existing claimants.
But a showdown could be avoided if support gathers instead around a motion of tabled by Church of England bishops that would allow the measure to pass but express "regret" at the "failure to take account of concerns about their short-term impact" and call for more consultation.
MPs have voted twice in favour of the cuts, but senior Tories including David Davis and London mayoral candidate Zac Goldsmith have joined forces with Labour's former welfare minister Frank Field on a cross-party motion calling for action to protect poorer families.
Although Thursday's Commons vote on a backbench debate is non-binding, a significant rebellion would heap further pressure on Mr Osborne to act.
Two former Archbishops of Canterbury - Rowan Williams and George Carey - are among 63 peers, 13 of them from the crossbenches, who have signed a letter to the Observer opposing the cuts in their present form.
"At a time when nearly two thirds of children living in poverty are in working families, none of this can be right," they say.
Tory former minister David Willetts also renewed his criticism of the policy - arguing that it was symptomatic of an economic policy skewed too far in favour of pensioners, an issue he has campaigned on for several years.
Mr Willetts, who stepped down as an MP in May and is executive director of the Resolution Foundation think-tank, said: "A pensioner couple with an income of around £15,000 can expect it to rise by roughly £300 as a result of the triple lock.
"This contrasts with Resolution Foundation estimates of a loss from tax credit cuts of around £1,500 for a family with one child in which a single-earner brings in that same £15,000 (and that's even after accounting for the welcome rise in the minimum wage promised by the Chancellor)."