Federal law bars service providers from turning over the contents of workers’ e-mail or text messages to an employer, even when the employer pays for the service, the U.S. 9th Circuit Court of Appeals ruled today.

The unanimous decision by the three-judge panel applies to all text messaging, and to all messages sent on a company e-mail that is handled by an outside provider. Employers who operate their own private e-mail systems will still be allowed to monitor their workers’ electronic communications.

The ruling provides 4th Amendment protection to text messages, which attorneys familiar with the case said would require police to obtain a warrant before they can access someone’s e-mail or text messages.

“This ruling is a tremendous victory for your online privacy, helping ensure that the 4th Amendment applies to your communications online just as strongly as it does to your letters and packages,” said the Electronic Frontier Foundation, a nonprofit group that advocates civil liberties in the digital world.

The ruling stemmed from a lawsuit by Ontario Police Sgt. Jeff Quon and three others against the city’s service provider and the city and Police Department for violating his constitutional rights to be free of unreasonable searches.

Although the city had informed employees that it had the right to read e-mails and text messages, the informal policy was that text messages sent over department pagers were not monitored for content, the court said.

If an employee exceeded the city’s limit on text messages, the employee had to pay the overage charges.

In August 2002, Quon and another officer exceeded the 25,000-character limit for texting. The police chief ordered a subordinate to obtain transcripts of the officers’ text messages to determine whether the pagers were being used purely for work purposes.

The provider, Arch Wireless, sent the department transcripts of the messages. The city determined that many of Quon’s messages were personal, and several were sexually explicit.

The court found that Arch Wireless violated the federal Stored Communications Act, which prohibits providers from divulging the contents of any communication which is maintained on the service.

Despite the fact that the Police Department paid for the Arch Wireless service, the court found, Quon and the other plaintiffs had a reasonable expectation that their messages stored on the service provider’s network were private.

“The extent to which the 4th Amendment provides protection for the contents of electronic communications in the Internet Age is an open question,” Judge Kim McLane Wardlaw wrote for the panel. “The recently minted standard of electronic communications via e-mails, text messages and other means opens a new frontier in 4th Amendment jurisprudence that has been little explored.”

The court declined to adopt a “monolithic view” of privacy involving text messages, saying a person’s expectation of privacy may depend on various factors. If Quon had permitted the department to review the messages, he would have no claim, the court said.

Instead, the Ontario Police Department “surreptitiously reviewed messages that all parties reasonably believed were free from third-party review,” the court said.

The audit of the messages, intended to determine whether a 25,000-character limit on texting was adequate for work purposes, was not reasonable, the court found.

“There were a host of simple ways to verify the efficacy of the 25,000 character limit [if that, indeed, was the intended purpose] without intruding … on 4th Amendment rights,” Wardlaw wrote.

The department could have warned Quon that for a month he was forbidden from using his pager for personal communications, and that all his messages would be reviewed, the court said.

If the department wanted to determine past usage, it could have asked Quon to redact all personal messages and permit the department to review the redacted transcript, the court said.

m aura.dolan@latimes.com