Greenpeace alleges that soy firms, including Cargill and ADM, have been encouraging local farmers to cut down the rainforest and plant massive soy monocultures.
The soy is then shipped to Europe, where it is fed to animals that are then turned into fast food products like chicken nuggets. The pressure group claims that last year more than 25,000 sq kilometres (10,000 sq miles) of Amazon forest were felled, largely for soy farming.
"This crime stretches from the heart of the Amazon across the entire European food industry," said Greenpeace forests campaign co-ordinator Gavin Edwards.
"Supermarkets and fast food giants, like McDonald's, must make sure their food is free from the links to the Amazon destruction, slavery and human rights abuses."
Greenpeace, which says that its allegations are based on a year-long investigation using satellite images, aerial surveillance and previously unreleased government documents, is hoping that this issue will have resonance within the growing ethical consumer sector.
Ethically sourced food is certainly becoming a consumer issue. A recent study by the UK's Co-operative bank suggests spending on ethical' food, including organic, fair trade and free range, was up from 3.7bn to 4.1bn in the 2004-5 period.
This prompted Melanie Howard, from the Co-op's research partner Future Foundation, to say the results should serve as a clarion call to business and government to take the upward trend in ethical consumerism very seriously.
And food companies in turn are starting to realise there is a win-win situation between business and ethics. If a company has a value-added long-term relationship with suppliers it can build a bond of trust, helping to overcome supply chain hiccups.
Cargill is certainly not taking this issue lightly. It says in a statement that it is committed to responsible economic development, which "enables responsible environmental management over time".
"In Brazil, as in all other countries where we operate, we take our corporate responsibilities very seriously.
"In the Santarem area, where we have a presence, we are working directly with farmers who are our suppliers to minimise negative impacts of soy production, and to maximize the economic benefits for families and communities.
"We are working with the global environmental group, The Nature Conservancy, in a pilot project to help our farm customers to employ the best agricultural practices in soy production and help them comply with local environmental laws."
But Cargill adds that while it is committed to encouraging sustainable business practices it alone cannot ensure sustainable soy development throughout Brazil.
Europe is an increasingly important market for Brazilian soy, partly because it is GM-free.
Brazil's 2005-06 soybean production could reach 57.4 million tonnes, according to the latest estimate from the Brazilian Vegetable Oils Industry Association (Abiove). Abiove's soybean export estimate for the 2006-07 is 24.5 million tonnes.