Save the Children is one of a long line of charities, think tanks, unions and leaders from across the political spectrum hitting out against the removal of the £20-a-week uplift.
The Government has been pressing ahead with the cut despite concerns that hundreds of thousands of people will be plunged into poverty.
From today, no assessments will include the uplift, meaning that from October 13 – a week later – no payments will be received that include the extra money.
The cut will be staggered over 31 days as families receive payments on different dates, Save the Children said.
Just over 3.5 million children in the UK are living in households that receive UC payments, according to Government figures.
This equates to 1.3 children being hit by the cut every second on average over the 31-day period from October 13, Save the Children said.
Rebecca, a single working mother, said she was already “feeling the pinch” and was concerned she would further struggle to feed her eight-year-old daughter when the uplift was removed.
She said: “What £20 means to families like mine is being able to afford proper healthy food – not just cheap processed food.
“It means having the heater on for two hours a day so we’re not freezing. This cut is going to be devastating for families such as ours.”
Gwen Hines, Save the Children’s chief executive, said children’s futures depended on the Government reinstating the lifeline.
She said: “Over the next month, every second that passes will see another child pushed towards poverty. People we work with tell us they’ve been relying on this £20 lifeline to buy essentials like food and clothing for themselves and their children. Without it, tens of thousands more children are facing a cold and hungry winter.”
She added that it was “astonishing” that the cut was going ahead, as families struggle with inflation, rising energy prices, fuel shortages and promised tax increases.
Helen Barnard, deputy director of the York-based Joseph Rowntree Foundation, said: “Today the Prime Minister has imposed the biggest ever overnight cut to social security.
“It makes a mockery of his mission to level up. Despite overwhelming opposition, he is ploughing ahead with a cut which fundamentally undermines the adequacy of our vital social security system as we face a cost-of-living crisis.”
Dr Mary Bousted, joint general secretary of the National Education Union, said the Government had ignored warnings and said the cut “contradicts the Government’s promise to level up”.
She said: “This step shows the Government is out of step with the rising cost of living for low-income families.
“Poverty creates concrete barriers to learning and we can’t rely on school improvement as the only strategy to address poverty.”
Rachel Harrison, GMB national officer, said: “Our care workers risked everything to keep our loved ones alive during the pandemic.
“The reward for many is to have their in-work benefits slashed by £20 a week. It’s a callous, cruel move from a chancellor who has no concept of the challenges working people face.”
Mark Serwotka, general secretary of the Public and Commercial Services Union, told PA: “The scrapping of the Universal Credit uplift is a scandalous decision driven by an ideological opposition to supporting the most vulnerable in society.
“Unemployed and low-paid workers both need access to Universal Credit to make ends meet and by removing the uplift thousands will be plunged into dire poverty, with people having to potentially choose between food and heating their homes this winter.
“There needs to be a fundamental overhaul of the social security system where no-one is left behind, regardless of their circumstances.”
The interventions come as Labour launches a poster campaign calling on the Government to cancel the cut in Manchester today to coincide with the Prime Minister addressing the Conservative Party Conference in the city.
Jonathan Reynolds MP, Labour’s Shadow Work and Pensions Secretary, said: “Today, the Prime Minister made a choice to cut the incomes of hardworking families. Any promise the Prime Minister makes to raise the living standards of people in this country rings hollow while this cut goes ahead.
“Under this government prices are up, bills are up and taxes are up, yet the Prime Minister has pressed ahead with the biggest cut to social security ever in the face of widespread opposition.
“Labour would maintain the uplift until we replace Universal Credit with a fairer social security system that lets people keep more of the money they earn.”
A Government spokesman said: “We’ve always been clear that the uplift to Universal Credit was temporary.
“It was designed to help claimants through the economic shock and financial disruption of the toughest stages of the pandemic, and it has done so.
“Universal Credit will continue to provide vital support for those both in and out of work and it’s right that the Government should focus on our Plan for Jobs, supporting people back into work and supporting those already employed to progress and earn more.”