Package delivery firm Purolator suspended acceptance of new shipments Tuesday after its largest union issued 72 hours' notice of a possible strike, which would commence Wednesday at 4:30 p.m. ET.

The move comes after a majority of the 8,162 Teamsters Canada members who work at Purolator rejected the company's most recent offer in negotiations.

Purolator's final offer has been rejected by members. Therefore, we've served a 72 hour notice of strike action to the company. —@TeamstersCanada

The company, which is majority owned by Canada Post, says it expects that shipments currently in the system will be delivered, but it will stop accepting new parcels as a precaution.

"We look forward to returning to the bargaining table, working diligently to negotiate a new agreement and getting back to delivering our customers' packages," the company said.

Purolator moves about one million packages or parcels a day, and took in more than $1.5 billion in revenue in its most recent fiscal year.

For its part, the union said a mediator has brought the two sides back to the bargaining table Tuesday, and they are now talking.

The two sides have been negotiating since last August to hammer out a new deal. According to a union release from December, the issues at the time included:

Contracting out routes to non-union carriers anywhere outside of a 50-kilometre radius of a depot

Changing the pension plan from defined benefit to defined contribution

Ending retirement bonuses

But in an email to CBC News, the company said none of those issues are currently on the table.

"We look forward to finding common ground with the employer," a spokesperson for the union said.