The justices ruled on a case that began in summer 2009, when two Boston officers patrolling in an unmarked cruiser rolled up to a car parked beside a fire hydrant in Jamaica Plain. The windows were down, and the officers noticed the driver light a small cigar commonly used to mask the odor of marijuana, according to court records.

“Our case law is clear that ‘the odor of marijuana is sufficiently distinctive that it alone can supply probable cause to believe that marijuana is nearby,’ ’’ Cowin wrote. “The advent of decriminalization certainly has had no effect on the distinctiveness of marijuana’s odor, nor has decriminalization affected the criminal status of numerous other activities involving marijuana.’’

“Even though possession of a small amount of marijuana is now no longer criminal, it may serve as the basis for a reasonable suspicion that activities involving marijuana that are indeed criminal are underway,’’ she wrote.

Justice Judith Cowin was the lone dissenting vote. She said that state law had allowed police officers to search a vehicle and order the occupants to exit if they smelled marijuana smoke.

Three years ago, voters overwhelmingly approved the ballot initiative decriminalizing the possession of small amounts of marijuana, with backers calling for a “more sensible approach’’ to marijuana policy and for law enforcement to focus on more serious and violent crimes. Opponents argued that the law would promote unsafe drug use.

He added: “In my mind, the odor of marijuana is clearly enough evidence of illegal activity to justify ordering those inside to exit the vehicle.’’

“I think this particular decision presents an enormous problem for us,’’ said Boston Police Commissioner Edward F. Davis. “The logic escapes me. . . . We will be the only state in the country where this standard is in place. It just doesn’t make a lot of sense.’’

“Ferreting out decriminalized conduct with the same fervor associated with the pursuit of serious criminal conduct is neither desired by the public, nor in accord with the plain language of the statute,’’ Roderick wrote.

The justices ruled that voters, in passing the 2008 ballot question, intended that possessing an ounce or less of marijuana “should not be considered a serious infraction worthy of criminal sanction.’’

“Without at least some other additional fact to bolster a reasonable suspicion of actual criminal activity, the odor of burnt marijuana alone cannot reasonably provide suspicion of criminal activity to justify an exit order,’’ the court ruled in a 5-to-1 decision written by Chief Justice Roderick Ireland.

The state’s highest court, overturning precedent and denying police a crime-fighting tool, ruled yesterday that the odor of marijuana smoke is not enough for officers to order a person out of a parked car, now that possession of less than an ounce of marijuana is no longer a crime in Massachusetts.

The officers said they smelled the “faint odor’’ of marijuana. One of the officers asked the driver if he had been smoking, and the driver replied that he had smoked pot “earlier in the day.’’ Neither officer saw anything illegal in the vehicle, but they said the driver and the passenger appeared nervous.

The officers called for backup and ordered the men out of the car, based on “the odor of marijuana and just the way they were acting,’’ they said.

One of the officers asked the passenger, Benjamin Cruz, then 19, whether he had “anything on his person,’’ and the young man responded that he had “a little rock for myself,’’ which turned out to be 4 grams of crack cocaine.

The justices cited three scenarios in which officers could order a passenger to exit a validly stopped vehicle: The officers felt they could be in danger; they had reasonable suspicion that the passenger was engaged in criminal activity; or they decided to search the vehicle.

The justices affirmed a district court’s ruling that the officers should not have ordered the passenger out of the car, because they “could not have reasonably feared for their safety considering the stop occurred during daylight, the defendant made no furtive or threatening movements, and the defendant was not known to the officers from previous arrests.’’

The justices said the officers had no evidence the men in the car shared marijuana or that they had a criminal amount.

“There are no facts in the record to support a reasonable suspicion that the defendant possessed more than 1 ounce of marijuana,’’ they ruled. “We conclude that, to order a passenger in a stopped vehicle to exit based merely on suspicion of an offense, that offense must be criminal.’’

One of Cruz’s lawyers — John Reinstein, legal director of he American Civil Liberties Union of Massachusetts, said he thought the justices answered a lingering question about police practices since the ballot initiative became law.

“The state took the position that the authority of police officers to conduct these kinds of searches was unaffected,’’ he said. “Ultimately, the question was whether the voters’ decision was to be given any meaning or whether we would go along with business as usual. I think the courts said . . . we have to give meaning to the new language of the statute.’’

Michael Cutler of the National Organization for the Reform of Marijuana Laws, which filed a supporting brief for Cruz, said the ruling underscored that personal use of marijuana “does not deserve attention and the expenditure of police resources that could be far better spent in terms of dealing with violent and predatory crime.’’

But prosecutors said they hope the ruling is more limited.

“The consensus is that this ruling applies to a very narrow fact pattern, in that the car was stationary when the officers approached,’’ said Jake Wark, a spokesman for the Suffolk district attorney’s office. “As a result, it’s too early to tell what effect [the new law] will have on when a moving vehicle is stopped and an officer smells smoke.’’

In a statement, Middlesex District Attorney Gerard T. Leone Jr., who helped lead opposition to the ballot initiative, said he agreed with Cowin’s dissent that the new law “does not affect the conclusions that may reasonably be drawn by police whose sense of smell alerts them to the fact that marijuana is or was present.’’

But he added: “Today’s decision sets a negative precedent for countless cases to follow. This is another example of the challenging legal ramifications for law enforcement.’’

David Abel can be reached at dabel@globe.com.

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