WASHINGTON – The Supreme Court ruled yesterday that police can have dogs check out motorists’ vehicles for drugs even if officers have no particular reason to suspect illegal activity.

The 6-2 opinion, written by Justice Stevens, stipulates police dogs may sniff only the outside of a car after a motorist is lawfully stopped for a traffic violation, such as speeding or failing to stop at a stop sign.

But privacy rights advocates said the ruling would lead to far more traffic stops as a way to find drugs. They also warned that the decision could open the door to more expansive searches, from sniffs inside the vehicle to checks of cars parked along sidewalks and pedestrians on the street.

Before yesterday’s ruling, the Supreme Court had authorized drug dogs primarily to sniff luggage at airports.

“The use of dogs is intimidating,” said Harvey Grossman, an attorney with the American Civil Liberties Union in Chicago. “Thousands of motorists have called complaining about suddenly finding their cars surrounded by policemen and drug dogs. Now no one is safe from this major intrusion into our lives.”

Illinois Attorney General Lisa Madigan, who argued the case, called the ruling a victory for law enforcement in the war on drugs. “The use of canine units to help fight this battle is indispensable,” she said.

The case involves Roy Caballes, who was stopped by Illinois police in 1998 for driving 6 mph over the speed limit. Although Caballes lawfully produced his driver’s license, troopers brought over a drug dog after noticing air freshener in the car and noting Caballes appeared nervous.

The dog indicated drugs were in the trunk, and police searched it even though Caballes refused to give permission. They found $250,000 worth of marijuana, and Caballes was convicted of drug trafficking.

The verdict was thrown out by the Illinois Supreme Court, which ruled the search was improper because police had no particular reason to suspect Caballes had drugs.

In his opinion, Justice Stevens reversed the state court ruling, saying the intrusion into Caballes’s privacy was too minimal to invoke constitutional protection. “A dog sniff conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess does not violate the Fourth Amendment,” Justice Stevens wrote.

In a dissent, Justice Ginsburg bemoaned what she called the broadening of police search powers, saying the use of drug dogs will make routine traffic stops longer and more adversarial. She was joined in her dissent by Justice Souter.

The court has long held that traffic stops should be brief since police often use them as a pretext to question motorists about other suspected illegal activity. Critics argue that authorities now will have wide power to check a car without consent, even if a police dog proved to be wrong about the presence of drugs.

“Under today’s decision, every traffic stop could become an occasion to call in the dogs, to the distress and embarrassment of the law-abiding population,” Justice Ginsburg wrote, citing the danger that police could soon conduct “suspicionless, dog-accompanied drug sweeps” of parked cars or cars stopped at red lights.

Justices Ginsburg and Souter noted, however, that they would not necessarily object in cases where dogs are needed to detect explosives or other hazardous weapons by a suspected terrorist.

Grossman said some states, including New Jersey, have passed legislation requiring police to have valid grounds to use dogs during traffic stops partly because of the danger of racial profiling, in which minorities are singled out for traffic stops or other scrutiny.

Mr. Grossman urged Illinois and other states to pass similar laws to protect motorists such as Caballes, who is Filipino.

The state of Illinois’s appeal was supported by 28 states and several law enforcement groups.

Chief Justice Rehnquist, who is ailing with thyroid cancer, did not participate in consideration of the case.