Boris Johnson faces a parliamentary challenge over the benefit cuts he has imposed on the country’s least well-off people, from a Tory peer who helped set up universal credit. Philippa Stroud, the chief executive of the rightwing thinktank the Legatum Institute and a former adviser to Iain Duncan Smith during his time as work and pensions secretary, said more than 800,000 people would be pushed into poverty, and threatened a vote in the House of Lords on the universal credit cut. “By our calculations, the decision today to remove this uplift will push 840,000 people into poverty – 290,000 of those are children – and so this is … a really bleak day for many, many families up and down the country,” she told BBC Radio 4’s Today programme. The prime minister has pushed ahead with his plan to remove the £20 uplift to universal credit, which was introduced at the beginning of the pandemic. The cut comes despite serious opposition – including on his own backbenches. Universal credit is available to many people in work, as well as jobseekers. Critics – including its architect Duncan Smith – have said the country’s poorest cannot afford the cut, which is being applied to assessments from Wednesday and will begin to take effect in a week, with the last families receiving the payment about a month later. Save the Children has warned that more than 3.5 million children in the UK are living in households that receive universal credit payments. Lady Stroud said: “There are people who are out of work who will move back into work, but there are also 450,000 who will move into poverty today as a result of this who have disabilities or who have children with disabilities. “It is not just people who are in employment or should be moving into employment who claim universal credit and I think we have to be really honest about who is claiming UC and why they’re there. “Our safety net is supposed to protect vulnerable people and that includes people who are sick, disabled and who have disabled children at this time.” She threatened to challenge the prime minister from the Lords, telling Today: “At this moment in time, MPs have not voted on this at all, it’s been a decision taken by the executive. So my intention is to bring a vote in the Lords, cross-party vote that would say to the House of Commons, think again on this issue. “Is this something we really want to do as a civilised nation? Putting our poorest people into poverty is surely not the way forward as we come out of the pandemic.” The deputy prime minister, Dominic Raab, defended the cut, telling Sky News: “Of course the emergency support we have provided was because of the pandemic. As we come through the pandemic, with youth unemployment going down, employment going up, we need to transition. We don’t want to see people reliant on the welfare trap.”