Plantations in Sri Lanka that supply "slavery-free" tea to top global companies are under investigation by ethical label groups after an expose found illegal wage deductions that have left some workers ill and unable to afford health care.

An investigation by the Thomson Reuters Foundation found that some workers at tea estates certified by Rainforest Alliance and Fairtrade were taking home as little as 26 Sri Lankan rupees (14 cents) a day after fees and deductions levied without consent.

Wage slips from 17 workers at nine Rainforest-certified tea estates — six of which were also backed by Fairtrade — showed daily earnings widely being cut by more than three-quarters for debt repayments, salary advances and a laundry list of fees.