Workplace computers contain so much personal information nowadays that employees have a legitimate expectation of privacy in using them, the Supreme Court of Canada said in a major ruling Friday.

The court said an individual's Internet browsing history alone is capable of exposing his or her most intimate likes, dislikes, activities and thoughts.

"Canadians may therefore reasonably expect privacy in the information contained on these computers, at least where personal use is permitted or reasonably expected," Mr. Justice Morris Fish said, writing for the majority.

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Notwithstanding that privacy interest, the court ruled that nude photos of an underaged students can be used to prosecute an Ontario high-school teacher who had them on his laptop.

It said that in some cases, the seriousness of an offence and workplace computer policies are sufficient to override the right to privacy.

Scott Hutchison, a privacy expert at Stockwoods LLP, said the decision will play an "incredibly important" role in stepping up the protection of computer privacy.

"For most people, the reality is that they use their work computers for personal matters in the same way they might make a personal call on a work telephone," Mr. Hutchison said.

While the judgment focused on principles rather than strict rules for workplace computer privacy, its words are bound to feature prominently in future cases.

"There is a big world of litigation coming about how employees are affected when employers try to go in and get information," said Frank Addario, a lawyer who represented the defendant, Richard Cole.

"Who doesn't use the Internet at work?" Mr. Addario said. "The Supreme Court is recognizing that the Internet is highly revealing of private personal choices and they have given it constitutional protection."

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Mr. Addario said future cases are likely to focus on the extent to which employers can scrutinize their employees' computer activity; whether a workplace policy could effectively eliminate an expectation of privacy altogether; and the constitutionality of attempts to obtain Internet search information from companies such as Google.

Employers will have to modernize their workplace computer use policies as a result of Friday's judgment, Mr. Addario said. He added that the court went out of its way to state that delving into an employee's Internet browsing history represents a clear incursion into his or her private world.

"There is no 'on-off' switch for privacy," Mr. Addario said. "Policies need to take into account the privacy interest that exists on the device and the information on it. It can't be invaded willy-nilly."

Mr. Cole was charged with possessing child pornography and fraudulently obtaining data from another computer hard drive after a school technician who was conducting routine surveillance found a file containing nude photographs of the underaged student.

The court ordered a new trial.

"The evidence is highly reliable and probative physical evidence," Judge Fish wrote. "The exclusion of the material would have a marked negative impact on the truth-seeking function of the criminal trial process."

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The student depicted in the photographs had sent them to another student by e-mail and Mr. Cole – whose responsibilities included patrolling students' e-mails and files – had allegedly gained access to it and copied them into his own computer.

The school's principal seized the computer, searched it more extensively and then provided it to police. Police investigators then searched the laptop and discs without a warrant.

The Supreme Court faulted the police for not having a search warrant. However, it said that their confusion was sincere and a warrant would have certainly been obtainable under the circumstances.