The Government has confirmed that weekly subsistence payments for victims of modern slavery are being cut by more than 40%.
The move was denounced by senior backbench MP Frank Field, who said it would "benefit slave drivers and exploiters" by forcing destitute people back into their clutches.
Coming just days after Theresa May hosted a summit of her Modern Slavery Taskforce at 10 Downing Street, the change will see financial support cut from £65 to £37.75 a week, but the period during which victims can claim the payment will be trebled from 14 to 45 days.
The Prime Minister has made tackling modern slavery a personal priority since coming into office, denouncing it earlier this week as "barbaric".
The Home Office said that there was no reduction in the total sums being spent on support for modern slavery victims, adding that the system was being brought in line with benefits for asylum seekers.
Financial support for dependent children of slavery victims will increase from £20.50 to £37.75 a week. Women who are pregnant or have young children will receive a one-off £300 maternity grant and an extra £5 a week for children under 12 months and £3 for those aged one to three.
Announcing plans for the change in a press release last October, the Home Office said it would significantly extend the "move-on" period during which victims receive support after leaving care, but made no mention of the reduction in weekly payments.
Mr Field called on Mrs May to halt the cut at Prime Minister's Questions in the House of Commons on Wednesday, telling her: "Organisations working with the victims of modern slavery report that tomorrow the Government will be cutting their miserable daily living allowance. Will the Prime Minister stop that cut?"
The Prime Minister responded that the Government had been "introducing changes in order to give more help to the people who need it most", but said she was not aware of details of how the changes would affect victims of modern slavery.
When the Home Office later confirmed the cut, Mr Field said: "This is an awful day for victims of modern slavery.
"Such a substantial cut, which the Government attempted to slip through unnoticed, will of course only benefit slave drivers and exploiters by shunting victims onto the streets and back into their clutches.
"This is surely not the legacy of the Modern Slavery Act that the Prime Minster had hoped for."
A Home Office spokesman said: "There will be no reduction in the total amount of money going to support modern slavery victims.
"Asylum seekers and victims of modern slavery often have similar essential living needs and so we are aligning the methodology used to calculate subsistence support we give to both, to ensure an approach that is consistent and fair."
The spokesman added: "All victims will also continue to receive dedicated and expert support tailored to their unique needs, including access to accommodation, legal aid, counselling, NHS medical and dental services."