The 9-0 ruling stripped police of an investigative power and gave Parliament 12 months to rewrite the law to fix a breach under the Charter of Rights and Freedoms.
The Friday ruling stemmed from an unusual case that reached the high court after the conviction of six men in a brutal kidnapping case in Richmond, B.C.
A British Columbia Supreme Court judge sentenced the men to prison terms ranging from 10 to 18 years in the February 2006 abduction of Peter Li, his wife Jennifer Pan and their friend Xiao Chang.
The RCMP started wiretapping immediately after they learned of the kidnapping and only obtained the necessary judicial authorization 24 hours later.
At trial, the judge ruled the police violated the Charter of Rights, but admitted the wiretap evidence anyway.
Such cases usually get to Ottawa only after a provincial appeal court hearing, but B.C. prosecutors sought and were granted leave to take the matter directly to the Supreme Court.
The Conservative government’s two new Supreme Court appointees, justices Michael Moldaver and Andromache Karakatsanis, co-wrote Friday's ruling. It struck down a section of the Criminal Code that allows police to intercept private communications without a warrant if they are trying to prevent an unlawful act that could cause serious harm.
The ruling made it clear that allowing police the power to intercept a private communication without a warrant does not inherently represent a charter breach.
But if the police are to have the power to install emergency wiretaps, the justices said Parliament must change the law to address the issue of accountability.
The ruling said Section 184.4 of the Criminal Code "falls down on the matter of accountability because the legislative scheme does not provide any mechanism to permit oversight of the police use of this power. Of particular concern, it does not require that notice be given to persons whose private communications have been intercepted."
Unless a criminal prosecution actually results from a wiretap, the targets may never learn that police were eavesdropping, depriving them of the ability to challenge the police use of this power, the ruling said.
The law failed to meet the minimum standard of Section 8 of the charter, which guarantees freedom from unreasonable search or seizure.
"After-the-fact notice … is one way of correcting this deficiency; it may not be the only one. Other effective means are no doubt open to Parliament," the ruling said.
A spokeswoman for Justice Minister Rob Nicholson said the ruling would be carefully reviewed to determine the government's next steps.
The RCMP declined to comment.
The NDP and Liberals both said the ruling sends a warning to the Conservative government over its proposed online surveillance law, Bill C-30. It proposes allowing police warrantless access to Internet subscriber information.
"It's a big blow the Conservatives' proposed Internet snooping bill," said NDP critic Jack Harris. "The court has expressed the same concerns as New Democrats raised from the beginning about this — the lack of accountability and threat to the rights and freedoms of all Canadians."
Liberal critic Francis Scarpaleggia said the ruling shows the court considers "Canadians' privacy rights to be sacrosanct, and that the government must properly protect these rights."
In the case at hand, the three kidnapped people were eventually released after the payment of a $1.3-million ransom, which was never recovered.
The six convicted men are appealing on a range of issues.
Their lawyers will likely make use of Friday’s ruling, but there is no direct, immediate impact on their case because of it.