Police powers to seize call records from tens of thousands of cellphone subscribers at once could be reined in with a court decision Thursday.

An Ontario judge will rule on a case brought by two of Canada's biggest telecommunications companies, arguing that sweeping "tower dumps" violate the Charter of Rights and Freedoms.

Investigators can seek a court order for a tower dump, compelling telecom companies to turn over the names and numbers of cellphone users whose signals were bouncing off a given cell tower over a particular time period.

​Police use tower dumps as an investigative tool so frequently that "hundreds of thousands, if not millions" of people across Canada are affected every year, Ontario Superior Court Justice John Sproat said in July 2014 when he agreed to hear the charter challenge by Rogers Communications Partnership and Telus Communications Co.

Sproat said at the time the case is the first in Canada to test the constitutionality of this police power and has "significant implications" for the relationship between law enforcement and telecom companies.

The ruling — due to be issued by Sproat at 11 a.m. ET Thursday — could limit the circumstances in which police can obtain court orders for tower dumps.

Telecoms make privacy argument

Lawyers jointly representing Rogers and Telus have argued police requests for tower dump records that cast too broad a net breach their customers' privacy.

"This technique involves the police accessing the records of a substantial number of individuals, virtually all of whom are not suspected of any wrongdoing or are even of interest to the investigation," lawyers Scott Hutchison and Christine Mainville said in their court submission.

"If not contained within reasonable parameters, these orders will allow police to obtain and retain potentially huge volumes of confidential information without proper grounds."

String of jewelry store robberies

The case currently before the Ontario court was triggered by a Peel Regional Police investigation into a series of jewelry store robberies in early 2014. Police obtained a court order for six telecom companies to provide customer information for all voice, text and data usage routed through at least 37 cell towers at specific time periods around the robberies.

Peel Regional Police wanted customer information for all voice, text and data usage routed through 21 cell towers to help investigate a series of jewelry store robberies in early 2014. (CBC)

Rogers and Telus asked a higher court to quash the order. Telus officials testified this was the "most extensive" police demand for customer data the company had ever received. Rogers estimated it would have to disclose the personal data of 34,000 subscribers, including names, addresses and billing information, such as bank account and credit card numbers.

The Crown acknowledged the size of the request in the probe falls "well outside the norm" of what police usually demand in tower dumps.

Peel Regional Police eventually withdrew the order, but the hearing into the constitutionality of the practice went ahead.

Lawyers for the attorney general of Ontario argued that tower dumps are a useful investigative tool for police when they have reasonable grounds to believe it will provide evidence of a crime.

The Crown urged the court not to impose tougher restrictions on tower dump requests than those that exist for wiretaps, or to cap the amount of information police can obtain.

Lawyers for the attorney general of Ontario are urging the court not to impose tougher restrictions on police requests for tower dumps than currently exist for wiretaps.

"Any search may gather information about innocent third parties," the province argued in its submission to the court. "Any suggestion that investigators should be required to use tower dump production orders as a tool of last resort should be rejected."

The lawyers for Rogers and Telus argued that tower dumps that are too broad in scope violate Section 8 of the charter, which protects Canadians against "unreasonable search and seizure."

Federal enforcement bodies make on average more than one million requests every year to telecommunications companies for private customer information, according to Canada's privacy commissioner.