Brazil is threatening to turn down Total's bid to drill in the mouth of the Amazon river.
The government environmental agency Ibama said in a statement late Monday that Total is required to "leave no doubt about possible impacts to the coral reef and the wider marine biodiversity."
The latest rejection of Total's environmental study puts at risk an ambitious bid by a group including Total, British Petroleum and Brazil's Petrobras to explore potentially vast oil reserves in the Amazon basin.
The 2016 discovery of the coral reef, which scientists and activists including Greenpeace say could be devastated if there were an oil spill in the river mouth, put the brakes on the project.
And Ibama says Total has still not done enough to overcome those concerns.
In the statement, Ibama warned that it "has already made three demands for more information in the environmental study" and that "if the investor again does not respond to the points raised by the technical team then the licensing process will be archived."
Total says the reef, which is 17 miles (28 kilometers) offshore, would not be in danger. It did not immediately respond to a request for comment on Ibama's threat.