Conservative peers will launch a last-ditch attempt to force the government to reverse the £20 a week universal credit cut on the eve of Rishi Sunak’s budget next week.

Philippa Stroud will table an amendment to the social security uprating bill in the House of Lords, with the support of Tory colleagues including the former work and pensions secretary David Willetts.

Constitutional precedent means it is usually considered unacceptable for the Lords to try to interfere in the financial aspects of legislation passed by the House of Commons.

But Lady Stroud – a close ally of the MP Iain Duncan Smith, who devised the universal credit system – is keen to highlight the scale of concern in the Conservative party about the cut, which has already come into force.

“I do not take lightly the idea of disagreeing so fervently with my Conservative government, or of breaking with parliamentary convention. But this cut is a grave misstep and risks undermining the levelling up agenda, leaving behind society’s most vulnerable people and putting at risk the stability of many homes up and down the country as we enter an unpredictable winter,” she said.

As well as Willetts, she is understood to have the backing of other Conservative peers including David Cameron’s former aide Gabby Bertin.

It is understood Lords clerks have warned Stroud they believe the amendment is out of the scope of the bill – but she still hopes to push it to a vote.

One seasoned Lords watcher said it had perhaps a one in five chance of reaching a vote. Stroud’s supporters point to a 2013 Lords vote on a boundary review that could have handed Cameron’s Conservatives 20 extra seats. The clerks objected on that occasion but peers voted nevertheless, delaying the review for five years and infuriating Cameron’s government. However, that issue did not have direct financial implications.

Liberal Democrat peers are supporting Stroud’s manoeuvre. Barbara Janke, the party’s work and pensions spokesperson in the Lords, said: “This cut hurts both those in and out of work, and risks pushing millions of families into deeper poverty. It’s encouraging to see that now even loyal Conservatives are taking a stand and opposing this decision.

“The Liberal Democrats want to see the £20 uplift retained and will be backing Baroness Stroud’s amendment, which we hope will receive cross-party support.”

Removing the £20 a week increase in universal credit that was introduced in the early days of the Covid pandemic amounted to the largest overnight benefits cut in history, according to the Resolution Foundation thinktank.

In his party conference speech earlier this month, Sunak said: “Picture this: you’re a young family. You work hard, saving a bit each month. But it’s tough. You have ambitions for your careers for your children. You want to give them the best – more than you had. Now you tell me: is the answer to their hopes and dreams just to increase their benefits?”

Labour has tabled a separate amendment to the bill – more likely to come to a vote – that would force the government to set out the impact of the cuts on pensioner poverty in six months’ time.