Ministers from the devolved nations have written to the UK Government calling for a reversal on a decision to cut the £20 Universal Credit uplift.
The increase was introduced during the pandemic, but is due to end on October 6.
Chancellor Rishi Sunak has previously defended the decision to go ahead with the cut to the payment, despite calls from charities and opposition parties to make the uplift permanent.
A joint-letter has now been sent by Scotland’s social justice secretary Shona Robison, Welsh social justice minister Jane Hutt and Northern Ireland’s communities minister Deirdre Hargey to the UK’s work and pensions secretary Therese Coffey.
In it, they state that the plan to end the uplift will cost people “over £1000 a year at a time when they need financial support the most”.
And they warn that the impact of the cut will be heightened by the “premature end” of the furlough scheme, as well as the rise in the price cap for default domestic energy deals.
They wrote: “This planned withdrawal will be the biggest overnight reduction to a basic rate of social security since the modern welfare state began, more than 70 years ago.
“It could cut social security payments in Scotland alone by over £460m per year by 2023/24.
“We are concerned about the potential impact that reducing Universal Credit will have on child poverty, poverty levels and the financial health and wellbeing of people.
“We urge the UK Government to reverse the decision without delay in order to avoid causing further anxiety.”
Last week, analysis by the Joseph Rowntree Foundation indicated that Glasgow contains the three Scottish constituencies which will be hardest hit by the cut to Universal Credit.
It said that more than half of families in Glasgow central, Glasgow South West and Glasgow North East would be affected.
Chris Birt, deputy director for Scotland at the Joseph Rowntree Foundation, said: “Cutting Universal Credit will have deep and devastating consequences for a huge number of families with children across Scotland.
“Child poverty was already rising before Covid-19; it is a scandal that the UK Government’s strategy for economic recovery is to plunge families who are already struggling into deeper poverty and debt.”