WASHINGTON – The Supreme Court extended the reach of a landmark federal disabilities law, ruling yesterday that foreign cruise lines that dock at American ports must provide better access for handicapped people.

The 6-3 ruling was a victory for disabled-rights groups claiming bias in the foreign cruise industry, which carries 7.1 million passengers each year.

The justices left it to lower courts to decide the extent of changes required under the Americans with Disabilities Act to prevent conflicts with international laws, which generally bar structural changes that would alter a ship’s design and threaten safety. Such changes could cost the industry many millions of dollars.

“The cruise lines aggressively market themselves as American and accessible, and maybe now they will truly become that,” one of the disabled passengers who sued the cruise lines, Douglas Spector of Houston, said. “Only then will they again earn back the trust of disabled travelers.”

The president of the Arlington, Va.-based International Council of Cruise Lines, Michael Crye, said his group would offer more personal assistance to the disabled, rather than making permanent, structural changes to its ships.

“We take pride in the strides that have been made, and will continue to improve the accessibility of cruise ships,” he said.

Justice Kennedy, writing for the majority, said Congress clearly intended for Title III of the ADA to apply to foreign cruise lines even though the 1990 law is silent on the issue.

“To hold there is no Title III protection for disabled persons who seek to use the amenities of foreign cruise ships would be a harsh and unexpected interpretation of a statute designed to provide broad protection for the disabled,” Justice Kennedy wrote.

But Justice Kennedy also said cruise lines need not comply with the ADA to the extent that it creates too much international discord or disruption of a ship’s internal affairs, under a provision of the statute that calls only for “readily achievable” modifications.

Illegal discrimination might include higher fares or surcharges for the disabled, or emergency equipment that is not readily accessible, Justice Kennedy wrote in an opinion joined by Justices Stevens, Souter, Ginsburg, and Breyer.

Justice Thomas agreed that ADA applied to the cruise lines, but he joined the dissenters in saying the modifications required under the federal law clearly did not extend to changes to a ship’s “physical structure.”

Much of the industry registers ships away from home countries in places such as the Bahamas, Liberia, Honduras, Panama, and Cyprus, which promote the practice by pointing to their business-friendly regulatory outlooks. The American cruise industry is almost exclusively foreign-flagged.

Three disabled passengers, who boarded a Norwegian Cruise Line ship in Houston in 1998 and 1999, say they paid premiums for handicapped-accessible cabins and the assistance of crew, but the cruise line did not provide restaurants, elevators, and other facilities that could accommodate them.

Norwegian Cruise Line countered that only an explicit statement of Congress can justify imposing American law on a ship that sails under a foreign flag, even if it is docked at an American port.