The decision will likely affect police monitoring of the Internet for purposes of detecting crime and terrorism, and courts may be overwhelmed by police requests for warrants.
The court ruled on a case in which Matthew Spencer, of Saskatchewan, was charged in 2007 for child pornography possession. He was located by police after his internet provider, Shaw Communications, voluntarily provided them with the IP address of the computer Spencer used and the street address of his sister, the contract holder with Shaw Communications.
The privacy Commissioner of Canada was among those who intervened in the case.
"This is a watershed moment for the right to privacy," the Criminal Lawyers Association said in a brief to the court. "An Internet service provider's records of its IP address assignments are like a cipher key that unlocks Internet privacy by linking individual subscribers to particular IP addresses on specific occasions. Someone armed with this information can easily learn details of a person's activities on the Internet, which can be extremely revealing."
The case is likely to be studied in other countries whose lawmakers seek to better define Internet and privacy rights.