Asda and Tesco have accused a newspaper and action groups of failing workers in Bagladeshi textile factories because they will not supply evidence of alleged abuses.
On Monday The Guardian reported on eight Bangladeshi workers who said they were routinely mistreated while working in factories used by Asda, Tesco and Primark.
They claimed they were forced to work 84-hour weeks and endure low pay and verbal and physical abuse. Reports by Action Aid and War on Want have made similar allegations.
Tesco and Asda said they were unable to investigate individual factories because these organisations, and The Guardian, refused to pass on their details.
"By not telling us the names of the factories, there is a degree of complicity as they are not protecting the workforce," said a spokesman for Asda.
But a War on Want spokesman said it had to keep silent because it feared supermarkets would withdraw business and leave vulnerable workers unemployed.
He claimed Tesco had pulled out of two factories in Bangladesh last October following a C4 exposé.
Tesco said the factories had been carrying out unauthorised sub-contract work.