WASHINGTON — The Supreme Court today struck down a federal law that forces private health organizations to denounce prostitution as a condition to get AIDS funding.

Chief Justice John Roberts, writing for the court, said the law’s anti-prostitution pledge improperly restricts the groups’ First Amendment rights.

Four organizations that work in Africa, Asia and South America challenged the 2003 law, arguing their work has nothing to do with prostitution.

The Obama administration argued it is reasonable for the government to give money only to groups that oppose prostitution and sex trafficking because they contribute to the spread of HIV and AIDS. It said that public money then could free up private funds that might be spent in a way that would undermine the government’s mission.

But the 6-2 decision said the pledge “goes beyond preventing recipients from using private funds in a way that would undermine the federal government.”

“It requires them to pledge allegiance to the government’s policy of eradicating prostitution,” he wrote. That, Roberts wrote, the government cannot do.

Justices Antonin Scalia and Clarence Thomas dissented. Scalia said the pledge “is nothing more than a means of selecting suitable agents to implement the government’s chosen strategy to eradicate HIV/AIDS.”

“That is perfectly permissible under the Constitution,” Scalia wrote.