The Taoiseach says Ireland will fight the Mercosur trade deal unless Brazil protects the Amazon rainforest.
The controversial deal would see almost 100,000 tonnes of beef from South America enter the EU every year.
The proposed agreement between the EU and a group of countries including Argentina, Brazil, Paraguay and Uruguay, was reached last month after 20 years of talks.
It has been heavily criticised by environmental groups and by beef farmers who believe it will lead to poorer quality beef flooding the EU market at the expense of Irish farmers.
Leo Varadkar has also criticised the Brazilian President for claiming that environmental groups were starting the fires to make him look bad, describing it as 'Orwellian.
"“There is no way that Ireland will vote for the EU-Mercosur free trade agreement if Brazil does not honour its environmental commitments," he said.
As global concerns rise over raging fires in the Amazon, Brazil’s government has complained that it is being targeted in smear campaign by critics who contend President Jair Bolsonaro is not doing enough to curb widespread deforestation.
Satellite images show a surge in fires since Mr Bolsonaro took power and promised to develop the region for farming and mining.
The threat to what some call “the lungs of the planet” has ignited a bitter dispute about who is to blame during the tenure of a leader who has described Brazil’s rainforest protections as an obstacle to economic development.
Brazilian federal experts reported a record number of wildfires across the country this year, up 84% over the same period in 2018.
Brazil contains about 60% of the Amazon rainforest, whose degradation could have severe consequences for global climate and rainfall.
Mr Bolsonaro, who has said he wants to convert land for cattle pastures and soybean farms, won office after channelling outrage over the corruption scandals of the former government.