Canada Post's union says the Crown corporation was irresponsible when it suspended urban mail operations across the country starting Wednesday.
The union said the decision by Canada Post's management means a large amount of mail already in the system will sit undelivered.
"Today, all postal workers were ready - the letter carriers as well - to distribute the mail everywhere in the country," union president Denis Lemelin said at a press conference.
"We were truly fulfilling our commitment to see to it that the public receives their mail."
Mr. Lemelin called for a meeting with Canada Post chief executive Deepak Chopra and called on him to commit to allow postal workers to deliver social assistance and other cheques.
Canada Post abruptly shut down operations across the country late Tuesday evening, locking out some 50,000 workers just as rotating strikes ended in Canada's two largest cities.
In a statement, the corporation said it had accrued almost $100-million in losses since the job action started and cited uncertainty for customers, a lack of progress at the negotiating table and "several incidents" that raised safety concerns.
"The instability is really causing our customers to have a serious doubt about our ability to deliver the mail, even if it's a rotating strike. This has pushed us to do a lockout as a way to really try to resolve this as quickly as possible," said Anick Losier, a spokeswoman for Canada Post.
Canada Post's decision - which affects urban centres but will slow mail to a trickle in rural areas - appeared to catch employees off guard. Members of the Canadian Union of Postal Workers said the corporation had not notified them of its plans in advance and workers were surprised to be blocked from facilities when they showed up to begin their shifts late Tuesday.
Union representatives said Canada Post's latest move appeared to be designed to get the government to legislate striking employees back to work.
"It's irresponsible for Canada Post to do that because Canada Post is a Crown corporation committed to give services to the population and we know there's mail in the system," said Denis Lemelin, CUPW's national president and chief negotiator.
The union has been staging rotating, single-day strikes in various cities and towns for 12 days. The largest work stoppages, in Toronto and Montreal, happened Tuesday.
The federal government had appeared to rule out back-to-work legislation earlier this week, but Labour Minister Lisa Raitt said Wednesday that she is considering bringing in legislation to end the strike.
The union has so far fiercely resisted key concessions.
To bring costs in line with a shrinking business model, Canada Post has proposed a two-tier compensation system that would see new employees get paid less than current employees ($19 an hour compared with $24 an hour), fewer days off and a less generous pension plan. To take advantage of new sorting equipment, the post office also wants to make changes to work methods for letter carriers that will allow it to get by with fewer workers in the future.
"Canada Post and CUPW remain far apart on several fundamental issues and there has been no progress made at the negotiating table for weeks," the corporation said in the statement. "We believe that a lockout is the best way to bring a timely resolution to this impasse and force the union to seriously consider proposals that address the declining mail volumes and the $3.2-billion pension deficit."
Ms. Losier declined to specify the incidents that raised safety concerns for Canada Post. Mr. Lemelin said that people may have been stopped and asked to respect picket lines, but said there had been no violence. "I think it's a false argument accusing the worker," he said.
Mr. Lemelin noted that pension and social-assistance cheques come out on Monday, which the union had an agreement to deliver. "We hope that Canada Post will fulfill this commitment," he said.
Canadians had already been feeling the effects of the labour dispute, not just from the rotating strikes, but because Canada Post had scaled back mail delivery in cities to Mondays, Wednesday and Fridays before declaring the lockout. The union had said the cutback was an attempt by the corporation to provoke a general strike.
The current country-wide suspension of urban mail is likely to have a wider impact.
Although the labour dispute does not include rural postal workers, who fall under a different contract, even the post office has acknowledged that a prolonged lockout could mean they would eventually little mail to deliver.
With a report from The Canadian Press