At the Canadian Imperial Bank of Commerce branch at Broadview and Danforth, a company poster hangs on a wall opposite a row of tellers. Hatched in the bank’s marketing department, the text is a deft bit of PR disguised as staff appreciation, thanking every member of the team for making CIBC number one in Toronto. Dara Fresco, the branch’s head teller, passes the poster several times a day. Though she greets customers with exceptional cheeriness—“Hi there! Nice to see you! How can I help you today?”—she has much less affection for her employer.

Fresco does not feel she’s been properly rewarded for her work, at least not financially, and feels she’s entitled to more than a thank you. For her scheduled 37.5 hours a week, she earns an annual salary of nearly $31,000—a figure just shy of what CIBC’s top boss, Gerald McCaughey, earns in a single day. But she wants to be paid for all the extra hours she spends performing tasks that she says can be done only once the bank’s doors are locked: balancing the ATM machine, counting inventory and finishing client paperwork. Often, she’d work overtime without compensation, and then get charged by her son’s daycare for picking him up late. She says she’s been “volunteering” her time to the company for more than a decade. The amount owing: $50,000 by her accounting.

Last June, Fresco booked a day off work and, under the glare of dozens of TV cameras in her lawyer’s Front Street office, announced she was launching a $600-million class-action lawsuit for unpaid overtime. As soon as she uttered her final thank you to reporters, the phone lines at her lawyer’s office lit up. Hundreds of fellow CIBC employees wanted in.

What followed was a wave of urgent e-mail messages and emergency meetings that went far beyond the banking sector. Bean counters and HR types hunkered down in boardrooms, combing through time sheets, worried that they, too, could face costly legal battles with their employees. With an estimated 1.6 million Cana­dians working unpaid overtime every month, the potential bill is in the billions.

Employee class-action suits are a new trend in litigation, one that’s moving north from the U.S. Over the past decade, Wal-Mart has faced dozens of lawsuits in dozens of states, including Pennsylvania, where in 2006, a court ordered the company to pay employees $78 million in unpaid overtime. That same year, IBM coughed up $65 million to settle a similar suit with 32,000 IT workers in California. Starbucks, UPS and Radio Shack have also forked out millions in retroactive overtime.

Historically, Canadian courts have not been as sympathetic to class-action suits as their U.S. counterparts. Recently, however, the Canadian judiciary has come to see them as a reasonably efficient way to settle complicated large-scale disputes, and an important means by which individuals in lower income brackets can gain access to the courts. Quebec was the first province to adopt class-action legislation in 1978; Ontario’s statute, the Ontario Class Proceedings Act, was finally enacted in 1992. Since then, Ontario has become a hotbed of class-action litigation, in part because the courts here are more open to the certification of national lawsuits.

To date, nearly 1,000 CIBC front-line workers (from a potential pool of 10,000) have come forward with stories similar to Fresco’s—how their employer neglected to pay them for time spent completing duties they claim they couldn’t possibly finish in their scheduled hours of work. Overtime for which, according to the CIBC policy book, they’re required to seek advance approval if they want to be paid. “It’s a booby trap that changes it from a right to a discretion,” explains Fresco’s lawyer, Doug Elliott, of Roy Elliott Kim O’Connor LLP. “To say, ‘You didn’t ask, so no, we won’t pay you overtime’ isn’t lawful.”