He spoke on a conference call as the group -- an independent, nonprofit alliance of more than 100,000 scientists and other activists -- presented its biennial report on the pollution performance of vehicles produced by the six largest automakers in the US market.

The report focuses on smog-forming pollution and carbon dioxide or heat-trapping gas emissions, the main pollutant linked to global warming. Passenger vehicles are blamed for roughly one-fifth of the smog-forming pollution in the United States.

The emissions performance of Honda was ranked the best for the third consecutive time and the Union of Concerned Scientists said its researchers found it builds vehicles that produce less than half the pollution of the industry average, when it comes to smog-forming emissions.

In global warming pollution Honda slipped from previous studies, however, producing just 18 percent less than the industry average.

Nissan Motor Co. Ltd. placed second in the rankings for the 2003 model year and Toyota Motor Corp. was third, followed by Ford Motor Co., DaimlerChrysler and General Motors.

"In sharp contrast to Honda, today we're naming General Motors public polluter No. 1 when it comes to emissions generated by automakers," said MacKenzie.

"GM has gone from being the best of the (Detroit) Big Three to the worst automaker overall in our rankings," he said.

The report said GM, the world's largest automaker, was the only one to produce vehicles that emit more smog and heat-trapping gases in the 2003 model year than they did in the 2001 model year.

GM spokeswoman Joanne Krell disputed the report's methodology, however, saying it penalized automakers who build more large trucks as part of their vehicle lineup.

"We think the report more accurately reflects an automaker's fleet vehicle mix, and US consumers' purchase behavior, than it does environmental performance," Krell said.

Nonetheless, MacKenzie said Ford's large trucks produced only half the smog-forming pollution emitted by large trucks from GM.

Tuesday's report came as the Washington-based Alliance of Automobile Manufacturers, which represents Detroit and many foreign automakers, said it had filed a federal lawsuit challenging California's tough new vehicle emission rules.

The move was widely expected and David Friedman, research director of the Union of Concerned Scientists' clean vehicle program, said such actions only add to the image problems already facing the world's most polluting car companies.

"Automakers have to stop trying to block environmental progress ... They should put engineers to work, not lawyers and lawsuits," Friedman said.