The decision to stop the review of a fungicide used on cereal, oilseed and vegetable crops violates the government's own legislation, said Lara Tessoro, a lawyer for Ecojustice, the firm acting for several groups behind the lawsuit.
"The duty on the government is to assess all the products containing the ingredient."
The lawsuit is over difenoconazole, which is known to be toxic to fish and believed by some scientists to accumulate in increasing amounts in the food chain. The suit was originally filed in 2013 in an attempt to force the government to review 23 different pesticides.
The action was put on hold after Ottawa agreed to the reviews. But the Pest Management Regulatory Agency has now pulled back.
"The special review ... is no longer required," said a letter from the agency to Ecojustice.
Canadian law requires a review for any pesticide banned in a member country of the Organization for Economic Co-operation and Development. Norway, which is a member, has banned difenoconazole.
But it is legal to import seeds that have been treated with the pesticide. That's enough, says the government, to remove the requirement for the review.
"In Norway, the use of difenoconazole is permitted on treated seeds," a department spokesman wrote in an email. "As Norway permits the use of difenoconazole, it does not 'prohibit all uses' of this product."
Tessoro said that's not how the Norwegians see it.
"Please note that this does not mean that we still have uses of difenoconazole allowed in Norway," says a letter to Health Canada from Norwegian officials. "It is prohibited to sell, stock, store or use difenoconazole in Norway."
Tessoro said Canada has ignored Norway's interpretation of its own law.
"Here's Norway telling Canada, 'No, no, don't be misguided here. We do not allow this pesticide to be used in this country.' Canada turned around and said, 'Thanks very much, Norway, but we're going to disagree with how you interpret your own laws.'"
She pointed out Canada's position is similar to one urged by the pesticide's manufacturer in a letter to the pest management agency after the review was promised.
Health Canada said it was unable to comment further on a matter before the courts.
The Norwegians say their decision was taken as a result of the chemical's "worrisome" toxicology. They say it tends to persist in the environment, concentrate in the food chain and is toxic to aquatic life.
Canadian assessments are similar. But in a letter to Ecojustice, the agency says the chemical's risks are well-managed through warnings.
"With the existing risk mitigation measures in place on the registered labels, the risks to aquatic species are not expected to be a concern," it says.
Ecojustice scientist Elaine MacDonald said difenoconazole is increasingly added to pesticides which some say are behind large die-offs in bee populations. She said the review her group is asking for would force Health Canada to examine all such combined products.
"I think it's worthwhile to have another look, evaluate the risk and evaluate the mitigation. That's all we're asking."
— Follow Bob Weber on Twitter at @row1960