Canada's postal service issued a plea on Friday for the rest of the world to stop sending in mail as its striking workers rejected the latest contract offer.
Canada Post, facing a huge delivery backlog as the labor unrest looked set to enter a fifth week, recently sweetened its offer to staff in a last-ditch effort to bring the rotating strikes to an end.
This followed a warning from Prime Minister Justin Trudeau that his government was prepared to step in to settle the labor dispute ahead of the upcoming holiday season.
His government has faced pressure from online retailers including eBay to legislate an end to the strike before the Black Friday and Cyber Monday sales events, which start on November 23.
But a spokeswoman for the Canadian Union of Postal Workers told AFP the offer, due to expire on Saturday, was "unsatisfactory" and the union "will not be presenting it to members."
Canada Post, meanwhile, said a backlog of deliveries that coincided with the start of the strike on October 22 has now extended to mail entering the country.
"As a result, we have been forced to advise international posts, including the United States Postal Service, that we are unable to accept incoming items until further notice," it said in an email.
The two sides have been in contract negotiations for nearly one year, with no success.
The rotating strikes have so far impacted more than 200 communities, including major cities Toronto, Winnipeg and Vancouver.
In Toronto alone, a record 260 trailers of parcels and packets were waiting to be unloaded, while in Vancouver more than 100 trailers were parked outside its plant.
Canada Post delivers two-thirds of the nation's online shopping and the last six weeks of the year are its busiest due to the holiday rush.