In the more than two years since the European Union proposed far-reaching new rules on chemical safety, manufacturers have argued that the burden of the regulations would kill off jobs. They are now making headway with another argument: It would kill animals.

The European Parliament tomorrow is expected to give preliminary backing to a chemicals-regulation proposal to compile health and safety dossiers on thousands of chemicals used in manufacturing.

The struggle over the law has made some animal rights groups into unlikely allies of the chemicals industry -- and given some cover to politicians who seek to downsize the regulations. Those groups argue that the expanded laboratory testing necessary to collect safety data would require as many as 3.9 million more animals to suffer or die in tests.

"We are not trying to hold animals hostage," said Alain Perroy, director of the European Chemical Industry Council, a Brussels trade group. But limiting the testing to only the most dangerous chemicals, as the Parliament is expected to endorse tomorrow, "will also save a lot of animals," he said.

The battle also has pitted animal rights activists against environmentalists who have championed the measure, though part of its aim is to protect animal habitats from chemical contamination. In Brussels, the World Wildlife Fund has led the charge for the European legislation, saying not enough is known about how chemicals affect the health of people and animals.

In response, People for the Ethical Treatment of Animals started a Web site called the Wicked Wildlife Fund to attack the nonprofit organization for supporting measures that PETA says would increase animal testing.

PETA is part of a loose coalition of animal-welfare groups working to scale back the legislation, known as Reach, for "registration, evaluation and authorization of chemicals." The British Union for the Abolition of Vivisection, for example, has lobbied against the proposal and on its Web site features a headline over a sad-eyed beagle that reads: "An animal dies in an EU lab every 3 seconds."

Given the high unemployment in many European countries, it is a testament to the influence of Europe's environmental movement that the chemicals proposal is moving forward in any form. But the animal rights lobby in Europe also is strong. By adopting the animal rights argument, politicians looking to scale back the plan have been able to avoid appearing merely pro-industry.

The result irritates some left-leaning Parliament members who want more safety data collected on chemicals. Asked if he thought industry lobbyists were exploiting the animal-testing issue for political advantage, Carl Schlyter, a Green Party parliamentarian from Sweden, said: "Of course. And why not? It helps them."

The United States, Japan, Canada, China, Australia, Mexico and other governments have complained to the E.U. about the original version of the legislation, arguing that applying stringent new chemical-testing requirements to imports as well as goods made in Europe would violate trade treaties and disrupt international manufacturing supply chains. The E.U., which has estimated the cost to industry of the original legislation at more than $5.85 billion over 11 years, has denied it would violate trade rules.

But last week, under heavy pressure from Germany and its chemicals industry, the two biggest party groups in Parliament -- Socialists and free-market European People's Party, both of which have Germans in key positions -- agreed to sharply reduce the amount of new safety data required on around 20,000 lesser-used chemicals, or two-thirds of the total covered. Green Party members are working to defeat the amended version in tomorrow's vote but have described their effort as uphill.

Meanwhile, industry-friendly politicians continue to play up the emotional animal-testing issue. It is "ethnically not acceptable" to pass a new law that would kill more animals, the E.U.'s industry commissioner, Guenter Verheugen of Germany, said at a recent news conference.

"I think there is a huge element of political opportunism in some of these statements," PETA spokesman Troy Seidle said. "But we don't mind, if it helps lead to an end to animal testing."

Garden gnomes were placed in front of the building of the European Parliament by environment group Friends of the Earth to help protest Reach, the European Parliament's chemical regulatory package. The package is expected to win initial backing this week.