Federal health officials won't put new restrictions on the use of a mercury-based preservative in vaccines and other medicines, denying a petition that sought the limits because of health concerns.

A group called the Coalition for Mercury-free Drugs petitioned the Food and Drug Administration in 2004 seeking the restrictions on thimerosal, citing concerns that the preservative is linked to autism. In a reply dated Sept. 26 but made public only Tuesday, the FDA rejected the petition.

"Only a small number of licensed and approved products still contain thimerosal, and the available evidence supports FDA's conclusion that all currently licensed vaccines and other pharmaceutical drug products containing thimerosal are safe," Dr. Jeffrey Shuren, the FDA's assistant commissioner for policy, wrote in denying the petition.

"We're not accepting that answer," said Dr. Mark Geier, one of the petitioners. The group now plans to seek a court order that would force the FDA to withdraw thimerosal from all vaccines and medicines unless the agency can show the preservative is safe, Geier said.

Thimerosal, about 50 percent mercury by weight, has been used since the 1930s to kill microbes in vaccines. There have been suspicions that thimerosal causes autism. However, studies that tracked thousands of children consistently have found no association between the brain disorder and the mercury-based preservative. Critics contend the studies are flawed.

Since 2001, all vaccines given to children 6 and younger have been either thimerosal-free or contained only trace amounts of the preservative. Thimerosal has been phased out of some, but not all, adult vaccines as well.

Most doses of the flu vaccine still contain thimerosal, though manufacturers produce versions free of the preservative for use in children. The FDA said it was in discussions with those manufacturers to increase the supply of thimerosal-free flu vaccine.

There also are minute amounts of mercury, as thimerosal or phenylmercuric acetate, in roughly 45 eye ointments, nasal sprays and nasal solutions, the FDA said. Various antivenoms for black widow and snake bites also contain thimerosal.