WASHINGTON (AP) — The Supreme Court ruled Wednesday that a state regulatory board made up mostly of dentists violated federal law against unfair competition when it tried to prevent lower-cost competitors in other fields from offering teeth-whitening services.

By a 6-3 vote, the justices rejected arguments from the North Carolina State Board of Dental Examiners that it was acting in the best interests of consumers when it pressured nondentists to get out of the lucrative trade in teeth-whitening services.

Justice Anthony Kennedy said in his majority opinion that the Federal Trade Commission was right to conclude that the state regulators also had a financial interest in the market for teeth-whitening.

In dissent, Justice Samuel Alito warned that the decision “is likely to have far-reaching effects on the states’ regulation of professions” because many boards are made up of practitioners.