LANSING, Mich. – A federal judge on Wednesday blocked a Michigan law that requires pedestrians under 21 to submit to a breath test without a search warrant.

The American Civil Liberties Union, which had sued on behalf of four college students, said the law is the only one of its kind in the country. U.S. District Judge David Lawson in Detroit ruled that it was unconstitutional to force non-drivers to submit to preliminary breath tests without a warrant.

“This is a tremendous victory for the civil liberties of young adults,” said Kary Moss, executive director of the ACLU of Michigan.

Under the 1998 law, pedestrians under 21 who refuse to take a breath test face a $100 fine. To require a breath analysis, an officer must have reasonable cause to believe that a minor has been drinking.

Backers of the law have said police need breath testers and other tools to enforce the legal drinking age.

In 2006, the city of Mount Pleasant and Isabella County agreed to pay $5,000 to two of the plaintiffs, Cullin Stewart and Samuel Maness, and stop warrantless pedestrian breath tests until Lawson issued a final ruling.

Both Stewart and Maness attended a 2003 post-prom party in Isabella County where, according to the lawsuit, an interagency police task force called the “Party Patrol” broke up the party, placed the students in a circle and asked if they had been drinking.

They had to blow into a portable breath tester, according to the suit. Stewart was not charged, but Maness was issued a citation accusing him of being a minor in possession of alcohol.

Michigan State Police, Central Michigan University and Saginaw County’s Thomas Township Police Department also are defendants in the case.

A telephone message seeking comment was left at the office of state police spokeswoman Shanon Akans.