The Supreme Court ruled yesterday that cruise ships must follow federal law that bars discrimination against disabled people, but left it for the lower courts to decide exactly what changes foreign-flagged ships, which make up the majority of vessels in U.S. waters, will have to adopt.

In a splintered ruling that produced opinions by no fewer than four justices, a six-justice majority agreed that foreign-flagged vessels plying U.S. waters could be required to eliminate barriers to disabled passengers. A plurality of three justices, Anthony M. Kennedy, John Paul Stevens and David H. Souter, defined that to exclude permanent changes to the "basic ship design and construction."

The case, Spector v. Norwegian Cruise Line, No. 03-1388, posed a conflict between maritime law, with its age-old notion that governments may not normally meddle in the "internal affairs" of foreign vessels, and the 1990 Americans With Disabilities Act, with its requirement that "places of public accommodation" and "private transportation entities" be made open and accessible for disabled people.

Major cruise lines are headquartered in Miami, ships often travel between U.S. ports, and 76 percent of the 9.8 million passengers who took cruises in 2003 were U.S. residents, according to the International Council of Cruise Lines.

But because the vast majority of those ships are registered under "flags of convenience" offered by the Bahamas and other "open-registry" countries, they are outside U.S. jurisdiction.

The case was brought by disabled passengers, backed by the Bush administration, who said they were charged higher ticket prices in 1998 and 1999 for accessible cabins aboard Norwegian Cruise Lines and that the swimming pools, elevators and bathrooms were not configured for wheelchair users.

David C. Frederick, a lawyer for Norwegian Cruise Lines, said the court had issued a "very favorable ruling that will ensure it is not going to have to retrofit ships, which is the thing that most concerned the industry."

Thomas C. Goldstein, a lawyer for the passengers, agreed that large-scale physical changes to ships will probably not be required, but that the ruling will lead to a requirement for accessible bathrooms, temporary ramps and the like.