Washington, D.C. ? The federal government must allow meatpackers to test their animals for mad cow disease, a federal judge ruled Thursday.

Creekstone Farms Premium Beef, a meatpacker based in Arkansas City, Kan., wants to test all of its cows for the disease, which can be fatal to humans who eat tainted beef. Larger meat companies feared that move because if Creekstone tested its meat and advertised it as safe, they might need to do the expensive test, too.

The Agriculture Department currently regulates the test and administers it to less than 1 percent of slaughtered cows. The department threatened Creekstone with prosecution if it tested all its animals.

U.S. District Judge James Robertson ruled that the government does not have the authority to regulate the test. Robertson put his order on hold until the government can appeal. If the government does not appeal by June 1, he said the ruling would take effect.

Mad cow disease is linked to more than 150 human deaths worldwide, mostly in Britain.