09/30/2011 04:14 EDT | Updated 11/30/2011 05:12 EST

CAW serves 72-hour strike notice served at St. Lawrence Seaway Management Corp.

ST. CATHARINES, Ont. - The Canadian Auto Workers union served 72-hour strike notice Friday for workers at the St. Lawrence Seaway Management Corp., raising the potential for a shutdown next week at one of North America's major shipping routes.

The two sides were expected to continue bargaining over the weekend with the help of a federal mediator but, if a deal is not reached, a strike by some 475 workers could start as early as noon on Monday.

If the unionized workers do go on strike, the St. Lawrence Seaway will be closed to all traffic, the company said in a statement Friday.

It added that a contingency plan provides for the orderly shutdown of the system in the event of a labour interruption.

The Seaway system allows for large freight ships to travel between the Atlantic Ocean and the five Great Lakes — serving some of the most important industrial cities in Canada and the United States and providing a route for the export of grain and other commodities.

The company is responsible for the movement of marine traffic through the Canadian Seaway facilities, which includes 13 of the 15 locks that lift and lower ships between Montreal and Lake Erie.

In Ottawa, Labour Minister Lisa Raitt urged both sides to reach an agreement noting that any work stoppage could hurt Canadian businesses and the economy.

"Our government is focused on completing the economic recovery and this means protecting Canadian jobs as the economy remains fragile," Raitt said in a statement.

Raitt threatened two unions at Air Canada with back-to-work legislation earlier this year, and legislated striking Canada Post workers back to work, citing the impact that the job actions would have on the economy.

CAW president Ken Lewenza urged Seaway management to be more responsive to union's concerns. He said that the 72-hour notice was necessary to get bargaining moving in the right direction.

The two sides began negotiating three collective agreements in May and the latest round of negotiations began Sept. 19.