Church Action on Poverty, Christians Against Poverty and more than 1,100 church leaders are calling on the government to abandon plans to cut Universal Credit.
A reduction of £20 a week will kick in from next month, amounting to £1,040 a year, with millions standing to be affected.
In their letter to Prime Minister Boris Johnson, the charities and church leaders warn of a devastating impact on people in poverty, leaving many to choose between heating and eating this winter.
"We urge the Government to choose to build a just and compassionate social security system that our whole society can have confidence in," the letter reads.
Christians Against Poverty's UK Chief Executive, Paula Stringer, said the cut would push thousands more people into problem debt, and force many to make "impossible choices".
"They'll be faced with the very real prospect of falling into arrears, and having to choose whether to eat or put the heating on to stay warm," she said.
She said the extra money was a "vital lifeline" for many people as she urged the Prime Minister to show compassion and make a last minute U-turn.
"No one should ever have to choose between food or heating, it's simply not right, but that will be the heartbreaking reality if this cut goes ahead," she said.
"Our message is clear - stop this before it cripples millions of households."
Niall Cooper, Director of Church Action on Poverty, said the cut could be "disastrous" for households already having to tighten their belts because of soaring food and fuel costs.
"We should be pulling together to get the economy back on course, but instead people are having to watch the threadbare lifelines being cut," he said.
"The breadth of support for this letter reflects the wider public's desire for a just and compassionate economy.
"We need to redesign the social security system so it brings stability and opens up opportunities, rather than sweeping families into deeper poverty."
Other signatories include bishops in the Church of England and former Vice President of the Methodist Church, Eunice Attwood.
She urged the Prime Minister not to overlook the impact of £20 a week.
"Every day we see in churches and foodbanks people with huge potential, bursting to contribute who cannot move forward because the struggle simply to make ends meet is all consuming," she said.
"I urge ministers to come to a church project and see the difference £20 a week makes for some families.
"We believe that everyone should be able to fulfil the potential God has placed within them and that we should do all that is necessary to prevent people being held back by poverty."
The Bishop of Doncaster, the Rt Rev Sophie Jelley, said she was concerned about the increase in food and energy prices coupled with the impact of the pandemic.
"There is no doubt that recent months have placed additional burdens on the poorest in our communities and this feels like the least we can do to show practical compassion and care at this tremendously difficult time," she said.
The Bishop of Selby, Dr John B Thomson, echoed concerns about the impact of the pandemic but he also expressed disappointment that the cuts are being made as furlough ends.
"I signed this open letter because this proposed cut comes at a time when the future of the pandemic remains uncertain and at the very point when the furlough scheme ends," he said.
"It will also coincide with significant increased costs for electricity and gas just when the weather begins to turn and is a concern which has been expressed widely by all organisations who work with the poorest, and those who monitor the impact of such policies on them.
"I accept that this is a major cost to the nation as a whole but believe that those in most need must be protected by the nation."