Textile workers in Bangladesh who make clothes for Primark, Asda and Tesco are paid as little as 7p an hour as living costs surge, a report suggests.
The charity War on Want said the workers are worse off than two years ago when it first exposed the abuse.
War on Want has urged the UK government to introduce laws to guarantee a living wage for overseas workers.
The retailers said they worked with their suppliers to ensure working conditions were of a high standard.
In its report, based on interviews with 115 workers from six garment factories during August and September, War on Want said high inflation and increasing fuel costs in Bangladesh had driven up the price of low-quality rice by 70%.
Prices of other cooking items, including oil, onions, pulses, wheat and flour, have soared by between 30% and 60%.
Workers said they needed £44.82 (5333 taka) a month to feed their families, pay for clean water, shelter, clothes, education, health care and transport.
But the charity said the average worker's wage was £19.16 (2280 taka) a month with the majority living in small, crowded shacks without plumbing and adequate washing facilities.
It said that by pressuring suppliers to keep the costs down, Primark was able to sell cheap fashion and carry on performing well despite the economic downturn.
'High ethical standards'
Denying allegations of exploitation, Primark said that by outsourcing garments to poor countries it was providing employment which would otherwise not be available, particularly for women.
"That fact alone has done more to empower women in the developing world than anything else, something that is all too frequently ignored by organisations keen to promote agendas of their own," the firm said in a statement.
It said its extensive auditing programme to ensure that suppliers complied with their contractual agreements meant that it could offer UK shoppers bargains, but "never at the expense of the people that make our clothes".
An Asda spokesperson said it was committed to the ethical treatment of its suppliers and would be happy to cooperate with War on Want to improve conditions.
"We would welcome the opportunity to work with War on Want to identify any issues and formulate a structured plan to help resolve this," she said.
Meanwhile, Tesco said the allegations were "unsubstantiated".
In a statement the UK's largest supermarket said: "We insist on high working condition standards, going to great lengths to ensure our suppliers meet them."