OTTAWA (Reuters) -The Canadian Teamsters union, which is negotiating a contract with the country's two largest rail companies, said on Friday a work stoppage could start on Aug. 22 if a deal is not reached.

"The Teamsters will provide 72 hours advance notice in the event of any strike action," it said in a statement.

The union, Canadian National Railway and Canadian Pacific Kansas City agreed last week to restart stalled contract talks with the aid of a federal mediator.

Canada relies heavily on its extensive railway network, given its expansive geography and exports such as grain, potash and coal, and a strike could cause significant economic damage.

In May, the federal government moved to push back a possible strike by asking the country's labor relations board to look at whether a stoppage might have safety implications.

The board announced on Friday that a strike would not cause immediate or serious danger to the public's health, a move that formally permitted the union to set a strike date.

In response, Labour Minister Steven MacKinnon said in a social media post that "it is the responsibility of unions and employers to negotiate deals at the bargaining table" and called on both sides to keep talking.