The U.S. Food and Drug Administration on Thursday will tell a federal judge in Kentucky that ordering the agency to delay enforcing new tobacco laws will have "devastating consequences" for public health.

The FDA is facing a challenge to its new tobacco powers, signed into law in June, from tobacco companies including Reynolds American Inc. (RAI) and Lorillard Inc. (LO). The companies say the law imposes unprecedented restrictions on their First Amendment rights and want a federal court in Bowling Green, Ky., to order a preliminary injunction to stop enforcement of the regulations.

A judge for the District Court for the Western District of Kentucky is holding a hearing on the preliminary injunction request and could soon decide whether to grant the injunction.

The law restricts tobacco companies from using color in most ads, bars them from saying certain products are less risky than others and stops them from selling tobacco products in combination with other items, such as soda and mouthwash.

"It is crucial to the public health that tobacco products not be marketed as reduced-risk products unless they will, in fact, reduce risks," the FDA said in a brief filed with the court.

The companies want to be able to make claims in ads and on boxes that certain cigarettes contain smaller amounts of harmful ingredients, such as being low in tar, and are, therefore, less risky than other tobacco products. In their briefing documents, the companies argue that such information is truthful and should be given to consumers.

The FDA says such information gives consumers the "mistaken belief" that the products are safe to use. The agency will allow companies to make such claims only after they prove the product does reduce a consumers risk for tobacco-related diseases. That appears to be a high hurdle. The agency notes that medical devices and prescription drugs must go through a rigorous review process before they can be sold to treat or reduce the risk of disease.

The companies say they aren't completely against the FDA authority to regulate tobacco, and that they support restrictions in marketing and advertising to children.

-By Jared A. Favole, Dow Jones Newswires; 202.862.9207; jared.favole@dowjones.com