WASHINGTON, Feb 23 (Reuters Legal) - The Supreme Court ruled that the existence of federal seat belt regulations does not bar state tort lawsuits seeking damages from automakers for installing lap-only seat belts.

The unanimous ruling held that a California lawsuit against Mazda Motor Corp over a fatal 2002 collision involving a 1993 Mazda minivan could proceed. A passenger sitting in a rear seat and wearing a lap-only seat belt was killed.

The lawsuit by the family of the passenger, Thanh Williamson, claimed that the minivan was defectively designed because it lacked a lap-and-shoulder seat belt for the rear seat.

Mazda argued that it was not negligent because it had complied with federal safety standards that provided it with the option of installing either lap belts or lap-and-shoulder belts on rear seats.

In 2008, the Court of Appeal of California sided with Mazda, affirming a lower court's ruling that said Williamson could not pursue a negligence claim because the car-maker had complied with federal regulations and that state-level claims were therefore pre-empted.

Justice Stephen Breyer, writing for the court, overturned the California ruling.

He stated that federal safety standards gave car makers an option and that the threat of tort suits limited that option. Critically, however, he added that there was no pre-emption because, "even though the state tort suit may restrict the manufacturer's choice, it does not 'stan[d] as an obstacle to the accomplishment of the full purpose and objectives' of federal law."

The ruling adopted the position argued by the Obama administration, which said the California and other courts have interpreted federal law too broadly in barring lawsuits against automakers that put in lap-only seat belts.

The vehicle safety regulations have been changed, and most passenger vehicles built after Sept. 1, 2007, include shoulder-and-lap seat belts in all rear seats that face forward.

The administration said the issue still was important and estimated that more than 1 million vehicles in the United States still have some lap-only belts.

Justices Sonia Sotomayor and Clarence Thomas wrote concurring judgments. Justice Elena Kagan did not take part in the ruling.

The case is Williamson et al v. Mazda Motor of America Inc, United States Supreme Court, No. 08-1314.

For Williamson: Martin Buchanan.

For Mazda: Mark Berry of Bowman & Brooke.

(Reporting by James Vicini of Reuters; Additional reporting by Jeff Roberts of Reuters Legal)