More than two decades ago, Congress adopted a sweeping law that outlawed female genital mutilation, an ancient practice that 200 million women and girls around the world have undergone. But a federal court considering the first legal challenge to the statute found the law unconstitutional on Tuesday, greatly diminishing the chances of it being used by federal prosecutors around the country.

A federal judge in Michigan issued the ruling in a case that involved two doctors and four parents, among others, who had been criminally charged last year with participating in or enabling the ritual genital cutting of girls. Their families belong to a small Shiite Muslim sect, the Dawoodi Bohra, that is originally from western India.

The case, the first to be brought under the 1996 law that criminalized female genital mutilation, has been closely followed by human rights advocates and communities where cutting is still practiced and whose members have moved in growing numbers to the United States and other western countries.

On Tuesday, Judge Bernard Friedman of the United States District Court for the Eastern District of Michigan ruled that Congress did not have the authority to pass the law against female genital mutilation and he dismissed key charges filed against the doctors and removed four of the eight defendants from the case.