The cut comes amid surging energy prices and “could have a catastrophic impact on individuals and families across the country”, national poverty charity Turn2us warned.
Its research also found that four in 10 (42 per cent) will be unable to afford food once the benefit is cut, leaving many with the choice between heating and eating as winter approaches.
A quarter of people also said they would be unable to afford rent or mortgage payments if the £20 is removed, and 15 per cent think it is likely they will lose their home in the next six months.
New figures show that record numbers of people are seeking support for their mental and financial wellbeing ahead of the widely opposed cut coming into force.
The charity is calling on the Government to keep the £20 increase to Universal Credit, which ends next week, and extend it to legacy benefits.
Anna Stevenson, of Turn2us, said: “At the start of October, Universal Credit will be cut by £20 at the same time as other support schemes like furlough come to an end and energy prices rise.
“Ultimately this will push many people who are struggling to cover basic needs like food, rent and utility bills, over a financial cliff edge.
“We know from our own data and insight, as well as our day-to-day work with people struggling to stay afloat, that this is a real cliff edge moment for thousands of people across the country.”