It appears contract talks at Canada Post have stalled as a strike mandate for more than 51,000 members of the Canadian Union of Postal Workers is set to expire.

The postal agency's biggest union has until midnight Thursday to serve the Crown corporation with a 72-hour strike notice.

The union is accusing Canada Post of continuing to make unacceptable demands.

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The added urgency of the strike mandate deadline saw federal Labour Minister MaryAnn Mihychuk directly intervene in the negotiations Tuesday by sitting at the table after the bargaining teams held days of intensive talks.

CUPW national president Mike Palecek has scheduled a news conference for Thursday to provide an update to the union's perspective of the negotiations.

But the union's bargaining team told members that, unless Canada Post backed away from its proposals, it would serve notice of an impending work disruption.

"If nothing changes between now and tomorrow, we will be issuing our 72-hour notice of strike activity," the CUPW negotiating team told its members.

Should the union pull the plug on bargaining, there could be disruptions to mail delivery by Monday, although it wasn't clear Wednesday what action would be taken.

Once the strike mandate expires, Canada Post employees would likely have to vote again on an extension.

That vote could take upwards of two months to complete, setting the timeline for a potential delivery disruption at the start of the busy Christmas mailing season.

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The Crown corporation also has the option to lock out workers after Thursday.

"After 9 months of negotiations, including 60 days of conciliation followed by more than 70 days with the help of mediators, both sides remain far apart," said Canada Post spokesman Jon Hamilton.

"The union continues to press for more than $1 billion in demands with no appreciation for the current and troubling future state of the postal service caused by declining mail volumes and increasing pension obligations.

"Canada Post remains committed to negotiating agreements that are fair to our employees, and allow us to continue to provide affordable pricing and service to Canadians."

A pay equity issue pitting rural and urban carriers against each other and a proposed move from a defined benefit pension plan to a defined contribution plan for new employees are the main stumbling blocks in the dispute.

The union claims rural postal workers earn, on average, nearly 30 per cent less than city carriers.

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"Canada Post's proposal on pay equity (for rural carriers) was nothing more than an attempt to complicate and delay that process," the union told its members in a statement.

"Canada Post wants to drag out pay equity with binding arbitration, a process that could take years or even decades."

Other issues include "precarious part-time and temporary employment, no improvements in staffing, the ability to close all 493 protected CUPW staffed retail locations eliminating up to 1,200 full-time jobs," the union said.