Workers on farms and plantations that supply big UK supermarkets are being subjected to poverty and human rights abuses, according to Oxfam.
A "relentless" drive for retailer profits is fuelling poverty, abuse, and discrimination, the charity said.
Poor conditions were rife on farms that supply supermarkets including Tesco, Sainsbury's and Morrisons, it added.
But the British Retail Consortium said retailers were "spearheading actions" intended to improve millions of lives.
Oxfam conducted research in India and Brazil, and surveyed workers in five other countries.
Workers on 50 tea plantations in Assam told Oxfam that cholera and typhoid are "prevalent because workers lack access to toilets and safe drinking water".
Half the workers questioned got ration cards from the government due to low wages, while female employees regularly worked for up to 13 "back-breaking" hours a day, it said.
Tesco, Sainsbury's, Morrisons and Aldi all source tea from those suppliers, while Asda-owner Walmart would neither confirm nor deny whether it did, the charity said.
Oxfam found that, of the 79p paid by shoppers for a 100g pack of black Assam tea in the UK, supermarkets and tea brands receive 49p while workers collectively received 3p.
The charity said workers on the Assam estates could earn a living wage if they were paid 5p more of the retail price.
Workers on fruit farms in Brazil told Oxfam they had developed skin conditions from using pesticides without adequate protection.
Women on those grape, melon and mango farms also said they had to rely on government handouts outside of harvest season.
Those farms supply supermarkets including Lidl and Sainsbury's, and previously Tesco and Morrisons, the charity said.
Walmart again neither confirmed nor denied links.
Rachel Wilshaw, Oxfam ethical trade manager, said: "Despite some pockets of good practice, supermarkets' relentless pursuit of profits continues to fuel poverty and human rights abuses in their supply chains.
"Supermarkets must do more to end exploitation, pay all their workers a living wage, ensure women get a fair deal and be more transparent about where they source their products."
A separate Oxfam survey of more than 500 workers in the Philippines, Ecuador, Costa Rica, Peru and the US found three quarters of workers saying they were not paid enough to cover basic needs such as food and housing.
More than a third said they were not protected from injury or harm at work and were not able to take a toilet break or have a drink of water when they needed it.
An Oxfam spokesperson said abuses in supermarket supply chains were "endemic".
However, Peter Andrews, head of sustainability at the British Retail Consortium (BRC), said: "Supermarkets in the UK are spearheading actions aimed at improving the lives of the millions of people across the globe who contribute to the retail supply chain.
"Our members are working hard to address existing injustices and continue to collaborate internationally with NGOs [non-governmental organisations], business groups and government on this vital issue."
Meanwhile, Oxfam ranked supermarket giants on their sourcing policies, with all showing an improvement compared with last year.
Tesco, which was at the top of the pile, was given a score of 38%.
A Tesco spokesperson said: "This is the second year in a row that Tesco has been assessed by Oxfam as doing most, of all major supermarkets globally, to ensure human rights are respected in food supply chains."
It said its tea was Rainforest Alliance certified and that it was "committed to improving the lives of tea workers and ensuring minimum working conditions."
It added: "We know there is always more to do and we are working collaboratively with NGOs, trade unions and others to improve wages in the key produce, tea and clothing sectors and ensure working conditions are fair."
An Aldi spokesperson said: "We continue to work hard to ensure every person working in our supply chain is treated fairly and has their human rights respected.
"We share the values behind Oxfam's campaign and are in regular dialogue with them."
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