Some of the UK's largest fast-food chains have been selling meat from animals fed on soya beans linked to Brazil's forest fires, campaigners say.
Some £240m of its soya was shipped to the UK in 2018, EU trade figures show.
Greenpeace said it wanted the companies to stop using soya from Brazil in their supply chains until the environment was better protected.
Brazil's environment minister told the BBC's Victoria Derbyshire programme any boycott could make the situation worse.
Some 2.5 million tonnes of soya is imported into the UK each year, with a large proportion used to feed farm animals.
In 2018, about a third of these - 761,739 tonnes - came from Brazil, BBC News analysis of the EU figures showed.
And just 14% of total soya imports are certified "deforestation free," according to the Sustainable Trade Initiative - one of the lowest rates in the EU.
Greenpeace head of forests Richard George said: "All of the big fast-food companies use soya in animal feed, none of them know where it comes from and soya is one of the biggest drivers of deforestation worldwide."
Environmental campaigners claim ongoing fires in both the Amazon and Cerrado regions of Brazil are being lit deliberately to clear land for raising animals and growing crops.
In 2019, the total number of fires surpassed 144,000, a 50% rise on the same period in 2018 - but far fewer than in 2010.
In 2006, Greenpeace and other environmental groups negotiated landmark restrictions on new soya cultivation in the Amazon, with huge agricultural traders agreeing not to buy from farms linked to recent deforestation.
But campaigners say that has pushed much of the problem south to the Cerrado, a vast tropical savanna where the natural habitat is less well protected.
Brazil's National Institute for Space Research recorded 19,925 fires there in September, significantly higher than the number in the Amazon.
In October 2017, 23 brands, including McDonald's, Tesco and Marks & Spencer, signed the Cerrado Manifesto, which recognised the need to prevent further deforestation.
But the agricultural trader Cargill, which acts as a middleman between farmers and food companies, has yet to sign up.
It is the largest importer to the UK, shipping 78% of the soya from Brazil in 2017, according to data from Trase.Earth, a partnership of non-governmental organisations. Although, Cargill told BBC News that figure was "not accurate and significantly inflated".
In July, Cargill told its Brazilian suppliers it would not support a temporary ban on soya grown on newly deforested land in the Cerrado - which has left environmental groups fuming.
"We remain committed to the soy moratorium in the Amazon but we believe that is not the right solution for the Cerrado," a spokeswoman said.
"Asking companies to exit won't solve the problem - it will simply move it. By pushing farmers to other buyers, the same practices will continue."
It has pledged $30m (£24m) to fund new ideas for ending deforestation.
A new analysis of satellite data by the Rapid Response project, seen by BBC News, suggests Cargill has been buying soya directly from farms in the region of the Cerrado responsible for forest fires. The project is a partnership between three groups, Aidenvironment, MapHubs and Mighty Earth.
On one farm, it says, 837 hectares (3.2 sq miles) of wooded savannah was cleared between April and June 2019 and fires were detected by the satellite imagery on 23 August.
It is impossible to say if crop from that specific farm was exported but trade data shows 7,103 tonnes of soya beans were shipped from the same municipality, Formosa Do Rio Preto, to the UK in 2017, the majority by Cargill.
Cargill accepts it does buy soya from the farm in question but says the farm met all compliance criteria and was not on the Brazilian government's embargoed list.
"As soon as we received an inquiry regarding potential non-compliance... we initiated our grievance process and an investigation is currently under way," a spokeswoman for the company told BBC News.
"We will take immediate action if illegal activity is found."
Environmental groups have been trying to increase the pressure on western retailers.
Tesco, Sainsbury and M&S have all pledged to achieve zero deforestation in their supply chains by 2020, although it is accepted that target is almost certain to be missed.
Environmental groups say the fast food sector is a particular concern - Burger King and KFC source some chicken directly from Brazil.
Along with chains such as McDonald's, Nando's, Pret a Manger and Five Guys they also sell British meat reared, at least in part, on soya shipped from the regions.
The proportion of animal feed made up of soya can vary dramatically between suppliers and farms in the UK, with some using a diet of grass and grain instead.
McDonald's says it is working to determine the level of deforestation risk in specific parts of the Cerrado and assessing whether fires are being lit at an individual farm level.
Other retailers and fast-food outlets, including Waitrose and Nando's, buy financial credits designed to offset the deforestation risk.
Mr George said: "This may sound persuasive but the actual soya they use may still come from farms that are destroying forests."
Burger King has been particularly criticised after pledging to end deforestation in its supply chains by 2030, a target criticised as "embarrassingly weak".
The company told BBC News it had written to its meat suppliers to remind them of its policy of not accepting products raised on former rainforest land. It says its beef suppliers in the UK use soya as a minor food additive only.
A spokeswoman said: "We are aware that in some of our beef, there are trace amounts of soya in the feed. We are also aware that there is no traceability programme in place anywhere in the world that can currently track all soy beans to a single farm in a single country."
Soya based foods like soya milk, tofu and meat replacement products are also made from soya beans, but the companies tend to be smaller in scale and more able to trace their supply chain.
The Vegan Society said: "The use of Amazonian soya in vegan food manufacturing is fairly insignificant, given that most brands source from Europe, and up to 91% of deforestation in the rainforest comes directly from animal agriculture. Farmed animals eat far more soya than we would if we ate it directly, therefore wasting resources and harming the environment."
The Brazilian government has faced intense criticism for policies environmentalists believe have encouraged the fires.
But Environment Minister Ricardo Salles told the Victoria Derbyshire programme pressure to shun Brazilian soya would be counterproductive.
"We need sustainable economic development... and boycotts or behaviours like this will only make things even worse," he said.