OTTAWA -- Canada Post said it has issued a 72-hour lockout notice to the Canadian Union of Postal Workers (CUPW), raising the spectre of a work stoppage by Friday.

The announcement came hours after Canada Post said its latest offer presented on June 25 was fair and reasonable and that it still hoped to negotiate a deal with the union.

But in a statement early Tuesday, Canada Post said it plans to suspend the collective agreement as of Friday. It blamed prolonged negotiations, the union's strike mandate and the financial cost of a rapid decline in mail volume.

Canada Post said the 72-hour notice does not necessarily mean it will shut down on Friday.

Rather, it said, the measure would allow it "take measures that are necessary to respond to the changing business reality."

In its response, CUPW said the corporation is using the lockout notice to drive 50,000 workers "out onto the streets pay in an effort to impose steep concessions on them."

"They refused to negotiate fairly with us and now they're locking the doors and will try to starve us into submission," said CUPW president Mike Palecek.

The two sides have both said the key sticking point in negotiations involves changes to employee pension plans.

The Crown corporation said Monday that CUPW's demands are "not affordable" and would add $1 billion in costs over the life of a new contract. The union accused Canada Post of preparing to lock workers out, and creating uncertainty by warning the public to avoid the post office.

Canada Post has said that in the event of a full work disruption, it will not operate -- mail and parcels will not be delivered, and no new items will be accepted.

The Canada Revenue Agency has deemed Old Age Security, Canada Pension Plan, Working Income Tax Benefit and the Canada Child Benefit cheques "essential" -- even during a labour disruption.

Spokesman Jon Hamilton said Canada Post has a memorandum of agreement with the union "where the federal socio-economic cheques will be delivered."

"In the event of a work disruption we would arrange ... delivery one day of the month," he said Monday.