Nearly 40 UK food businesses have threatened to stop sourcing products from Brazil over proposed land reforms.
An open letter from the group calls on Brazil's legislature to reject a bill which could legalise the private occupation of public land.
The letter said the proposal could accelerate deforestation in the Amazon.
The bill is being considered just months after Brazil pledged to end illegal logging.
Sainsbury's, Aldi, Greggs, the Co-Op, the British Retail Consortium, and the Hilton Food Group are among the major organisations to sign the open letter.
A vote in the Senate on the bill is expected within days.
'Vital part of earth system'
The companies say they "consider the Amazon as a vital part of the earth system that's essential to the security of our planet, as well as being a critical part of a prosperous future for Brazilians and all of society."
Rainforests are critical to mitigating the effects of climate change, as they store vast amounts of carbon.
Under the leadership of right-wing President Jair Bolsonaro, the level of deforestation in the Amazon is reported as being the highest since 2008.
This year alone around 430,000 acres of the Amazon have been logged or burned, according to the Monitoring of the Andean Amazon Project.
The vast majority of land is cleared either to graze cattle for beef exports, or to grow soy, which goes in to animal feed around the world.
At a summit in April hosted by US President Joe Biden, Mr Bolsonaro declared that Brazil would end illegal logging. The letter says these measures "run counter" to this "narrative and rhetoric."
The new law would allow land that has been illegally occupied after 2014 to be put up for sale. This would potentially allow illegal occupants to buy it.
Similar controversial measures were first put forward in a different bill last year, but were withdrawn after more than 40 organisations made the same threat over supply chain sourcing.
The group says the existing protections and land designations have been "instrumental" in their organisations having "trust" in Brazilian producers.
The companies say that the "door remains open to work with Brazilian partners" to develop sustainable land management practice in Brazil.
However, if this or other measures that undermine existing protections become law, they will have "no choice but to reconsider our support and use of the Brazilian agricultural commodity supply chain."
Commenting on the imminent vote, Mike Barrett, executive director of science and conservation at WWF-UK, said: "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."
Cathryn Higgs, head of policy at the Co-op, said it was "imperative" the proposed legislation wasn't "given any airtime by the Brazilian government."
Supporters of the bill say it would help small-scale farmers to clarify title deeds to land.
The Brazilian government has been approached for comment.
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