Supporters said it would enable the regulation of GM crops in Mexico and an evaluation of any possible risks to human health and the environment.

Opposition to the law came mostly from the leftist Party of the Democratic Revolution, which claimed, along with environmental groups like Greenpeace, that the new law would endanger corn diversity in Mexico, the birthplace of the grain.

"It's important to make clear that we are not completely satisfied with the result, given it does not express many of the demands we come across in the course of our work," said PRD deputy Jose Luis Cabrera.

Greenpeace has called the new legislation the "Monsanto Law," claiming it protects the economic interests of the multinational producer of genetically modified crops from policies that could cut into profits.

"This only benefits multinationals and supports the interests of a tiny elite in Mexico and goes against thousands of farmers," Greenpeace spokeswoman Cecilia Navarro told local media after one of the group's activists briefly entered the debating chamber in Congress.

A NAFTA environmental panel from Canada, the United States and Mexico recommended in October that Mexico adopt strict measures to control imports of genetically modified corn.

One recommended measure, which could only be carried out at considerable expense to companies like Monsanto, was that corn be milled at the border, before entering Mexico, in order to prevent contamination of its 7,000-year-old corn gene pool.

In recent weeks the report was attacked by US authorities as "fundamentally flawed and unscientific," and Mexican trade authorities said they had no plans to change import policies.

Mexican farmers say they need to stop imported corn that is genetically modified from mixing with local strains.