Police in New Jersey will soon have to get warrants if they want to track suspects using cellphone data, the state's Supreme Court ruled in a decision that affords citizens here more privacy protections than they enjoy under federal law.

In a unanimous ruling Thursday stemming from the arrest of a burglary suspect in 2006, the court directed that beginning in 30 days, all law enforcement officers must get a search warrant based on probable cause if they want to get access to cellphone locating data. Since 2010, police have had to satisfy a lower standard of demonstrating there are "reasonable grounds" to believe the information would be relevant to an investigation.

"No one buys a cellphone to share detailed information about their whereabouts with the police," Chief Justice Stuart Rabner wrote. "That was true in 2006 and is equally true today. Citizens have a legitimate privacy interest in such information."

Rabner noted that federal courts have been divided over the issue of cellphone tracking by law enforcement. In some other areas, he wrote, New Jersey's constitution goes farther than the Fourth Amendment in protecting citizens from unreasonable search and seizure — particularly in previous cases involving internet usage, bank records and hotel telephone records.

"When people make disclosures to phone companies and other providers to use their services, they are not promoting the release of personal information to others. Instead, they can reasonably expect that their personal information will remain private," Rabner wrote. "For those reasons, we have departed from federal case law that takes a different approach."

Rubin Sinins, an attorney who argued on behalf of the American Civil Liberties Union of New Jersey and a state criminal defense lawyers association, called Thursday's decision "vitally important."

"I'm not surprised it was unanimous because the basic premise of the opinion is quite logical and consistent with citizens' reasonable expectation of privacy in their cellphone usage," he said.

In the 2006 case, police tracked Robert Earls to a motel on Route 9 in Howell using information provided by T-Mobile about the location of a cellphone believed to be in his possession. When he opened the door to his room, police saw items they believed had been stolen and arrested him. He eventually pleaded guilty to burglary and theft.

It wasn't immediately clear how Earls' case would be affected by the ruling since a lower court will now have to consider whether police were justified in using the cellphone data without a warrant under an exception for emergency circumstances. Police said they believed Earls' girlfriend was in danger because she had cooperated with them.

The new warrant rule applies only to Earls' case and future cases. The state attorney general's office has trained county law enforcement personnel to obtain warrants for GPS-based location data since 2006, and warrants were obtained in about 85 per cent of 600 cases involving cellphone location data for a six-month period last year, according to Thursday's ruling.

"As a practical matter, this ruling only affects future cases, and police in New Jersey already have been routinely seeking probable-cause based warrants before seeking cellphone location information," attorney general's office spokesman Peter Aseltine said in an email. "We will implement training for New Jersey law enforcement to ensure compliance with this ruling."