The country's largest postal workers union has given Canada Post a 72-hour notice of job action, meaning mail and parcel delivery could be affected by work to rule conditions by Monday.

The Canadian Union of Postal Workers (CUPW) threatened job action, which could include work to rule, after Canada Post president and CEO Deepak Chopra refused to accept a 24-hour extension for contract talks during a meeting with Labour Minister MaryAnn Mihychuk Wednesday night. The union had until midnight Thursday – when its strike mandate was set to expire – to issue a notice of job action.

"For months, Chopra has been trying to provoke a disruption in the middle of the government's review of Canada Post. Unfortunately, Mr. Chopra's latest actions leave us no choice but to file a 72-hour notice of job action," CUPW national president Mike Palecek said in a statement Thursday. "We are still willing to withdraw our notice if Canada Post agrees to an extension."

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CUPW said it listed its anticipated job actions in a letter to Canada Post management. While the union does not plan to walk off the job, work to rule is an option.

The notice of job action is the latest in the very public and tense contract talks between Canada Post and CUPW.

It came hours after the federal government announced that it will appoint a special mediator to help with the contract negotiations and avoid a work stoppage. A mediator has not yet been named, but Ms. Mihychuk said she will work "expeditiously" to appoint one.

"I expect both parties to work with this special mediator to come to a resolution and avoid a work stoppage. I continue to closely monitor the situation," Ms. Mihychuk said in a statement Thursday.

The union said it welcomes the help of special mediator and is waiting to see who the minister will appoint.

Canada Post said it will fully cooperate with the special mediator and hopes the "assistance of a neutral third party will help both parties address the real challenges facing the postal service caused by declining mail volumes and increasing pension obligations."

CUPW is Canada's largest postal union, representing 50,000 postal workers across the country.

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The two sides continue to remain far apart in the dispute, especially on pay discrepancies between rural and urban mail-carriers and proposed changes to the pension plan.

CUPW wants rural and suburban mail carriers to be paid by the hour, like urban letter carriers, rather than by the number of parcels they deliver.

Canada Post wants to change the pension plan for new employees to a defined contribution plan, which does not pay a guaranteed return in retirement, instead of a defined benefit plan, which workers earn a set pension regardless of how the fund's investments do. Many private sector companies have moved towards defined contribution plans.

Earlier this month, an arbitrator accepted Canada Post's proposal to reach a new collective agreement for the Canadian Postmasters and Assistants Association, which represents 5,000 rural workers, that included a move toward a defined contribution plan. The decision makes CUPW the only remaining postal group that has a defined benefit plan for new employees.

If the union goes ahead with its threat of job action, mail and parcel delivery could be disrupted by Monday. In the event of a labour disruption, Old Age Security, Canada Pension Plan, Working Income Tax Benefit and Canada Child Benefit cheques would still be delivered because they are deemed essential.

Canada Post workers have been without a collective agreement for months; its contract with rural and suburban mail carriers expired on Dec. 31, 2015, and urban mail carriers on Jan. 31, 2016.

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This is not the first time a labour stoppage has been threatened. In July, Canada Post withdrew its 72-hour notice one day before it was due to lockout the CUPW members, which would have interrupted postal service.

As Canada Post continues to negotiate with its largest union, it is facing financial challenges stemming from a significant drop in letter mail and competition with the private package delivery business, such as UPS and FedEx.

With files from The Canadian Press