One of Brazil's most senior lawyers has appealed to the Supreme Court to overturn a government decision which allows farmers to grow genetically modified crops.
Environmentalists say GM crops will compromise Brazil's integrity
Last week, a government decree permitted the planting of genetically modified soy beans for one year in the south of the country.
But Procurator-General Claudio Fonteles, a senior official who rules on the legality of government policy, has now joined the Green Party and the Confederation of Rural Labourers in challenging the decision.
Mr Fonteles argues that ministers acted unconstitutionally by introducing the policy in the form of a decree - a charge the government rejects.
The procurator-general also says the government ignored an earlier court ruling which said there should be an independent study of the issue before planting could begin.
In its defence, the government points out that this is a limited measure under which genetically modified soya beans will only be planted in the south of the country and only for one year.
Brazil's vast agricultural tracts are a tempting prospect for biotech companies
The BBC's Steve Kingstone in Sao Paulo says the government's controversial decision has prompted strong criticism from environmental campaigners.
But our correspondent says Mr Fonteles' challenge is the most significant of three legal challenges from groups and individuals opposed to the measure.
A second comes from the country's Green Party, which says the government has a duty to guard Brazil's environmental integrity.
Also seeking to block the move is the Confederation of Rural Labourers. It says the planting of genetically modified crops will adversely affect the working lives of 15 million people.
Brazil was one of the last of the world's major agricultural producers to ban GM crops.
And even now, correspondents say, many of its soya farmers have simply ignored the ban - smuggling illegal seeds in from neighbouring Argentina for cultivation.
Some have estimated that one-third of Brazil's soya fields are given over to illegal GM crops.