A local ordinance that requires warnings about cell phone safety risks should be blocked from taking effect, an industry group argued in a U.S. court filing on Tuesday.

Health questions about cell phone use ramped up earlier this year after a working group of cancer experts from the World Health Organization suggested it should be classified as possibly carcinogenic. Industry groups maintain that does not mean that cell phones cause cancer.

San Francisco's Board of Supervisors revised a cell phone disclosure law in July. Among the regulations, retailers must display a large poster stating, in bold, that studies continue to assess potential health effects of mobile phone use.

Retailers are required to comply with the poster rules by October 25.

In a court filing on Tuesday, the wireless industry group CTIA argued that San Francisco's ordinance is an illegal speech restriction that is preempted by federal law.

Enforcement of the city's regime is imminent and will cause direct and irreparable harm, the group wrote in its request for an injunction.

San Francisco City Attorney Dennis Herrera said cities have a vital interest in keeping people informed about health issues.

I'm disappointed that the wireless industry is so bent on quashing the debate about the health effects of cell phone radiation, Herrera said in a statement.

An industry representative could not immediately comment on Tuesday.

A hearing on CTIA's request is scheduled for October 20, according to Herrera's office.

The case in U.S. District Court, Northern District of California is CTIA-The Wireless Association v. The City and County of San Francisco, California, 10-cv-3224.

(Reporting by Dan Levine; editing by Carol Bishopric)