Competition Bureau Commissioner Melanie Aitken is attempting to thwart Air Canada’s joint venture with United Continental on routes between Canada and the United States, warning it creates a monopoly that will raise airfares.

The airlines agreed last fall to form a partnership on transborder flights, sharing information on schedules, sales and pricing. United Continental Holdings Inc. and Air Canada say coordinating their routes allows consumers to take advantage of “more travel options and benefits.”

But Canada’s Competition Bureau says the joint venture is “effectively a merger” of the carriers’ Canada-U.S. operations.

The agency and the airlines have met several times to try to reach an agreement, but in the end, Aitken opted to seek an order blocking the partnership rather than continue to search for alternatives. The bureau filed an application Monday with the Competition Tribunal.

The case is the latest example of the bureau’s tougher stance under Ms. Aitken’s leadership, and follows other aggressive moves against the Canadian real estate industry, credit card companies, gasoline cartels and cellphone providers – solidifying her reputation as a steadfast protector of consumer interests.