Tesco, Aldi, Marks & Spencer, Lidl, Sainsbury's, Co-op, Waitrose and Iceland are among the major retailers to have raised concerns over a new effort by Brazil to legalise the private occupation of public land, mostly in the Amazon.
Asda, Morrisons and Greggs have also signed an open letter warning against the proposed law, along with other retailers, producers, investors and industry bodies.
It is feared Brazil’s plans, which were first attempted a year ago but was withdrawn after more than 40 major organisations made the same threat over supply chain sourcing, will drive deforestation.
Conservationists have previously warned it will legitimise previous illegal land grabs and pave the way for more forests to be burned and cleared for agriculture such as beef and soy, which is used as an animal feed for livestock.
It will make it harder to meet international targets to tackle climate change, as forests are huge carbon stores, and undermine the rights of indigenous communities in the region.
In the open letter to Brazilian politicians, the businesses said the new law being put forward for a vote posed "potentially even greater threats to the Amazon than before".
The companies warned that in the past year they had seen circumstances "result in extremely high levels of forest fires and deforestation in Brazil" and that the targets and budgets to reduce the problem were inadequate.
Existing protections and land designations enshrined in Brazilian law help companies have trust that their products and investments are aligned with the commitments they have as environmentally and socially responsible businesses, they wrote.
The companies say they want to help develop sustainable land management and agriculture in Brazil, and support economic development while upholding indigenous community rights, without putting at risk progress in protecting vital natural systems that are essential for the world.
"However, if this or other measures that undermine these existing protections become law, we will have no choice but to reconsider our support and use of the Brazilian agricultural commodity supply chain," the letter warned.
Cathryn Higgs, head of food policy at the Co-op, said: "The Amazon faces a new threat with legislation that undermines the credibility of environmental protections.
"Its rainforest is essential to planetary health and it's imperative the proposed legislation isn't given any airtime by the Brazilian government.
"We are joining forces with environmentally and socially responsible organisations to oppose the measures being put forward. If these new laws are brought in, we will have no choice but to reconsider our support and use of the Brazilian agricultural commodity supply chain."
Mike Barrett, executive director of science and conservation at WWF-UK, added: "We cannot fight the climate crisis without the Amazon, yet its future hangs in the balance as deforestation pushes it closer to the point of collapse.
"If passed, this vote in the Brazilian Congress will fuel further destruction and place greater risk on the lives of the people and wildlife who call it home.
"As global efforts to protect the Amazon threaten to be undermined, it's encouraging to see major businesses sounding the alarm."
Additional reporting by PA
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