The union representing some 48,000 postal workers will begin a series of rotating 24-hour strikes Friday morning, beginning in Winnipeg, after failing to negotiate a new contract with Canada Post.

The Canadian Union of Postal Workers said it would begin job action in the Manitoba capital, then announce where the next strike would take place. A press conference is expected at 9 a.m. ET.

Canada Post offered one last concession Thursday before the 11:59 ET strike deadline, but it was not enough for the union.

The government-owned corporation said it was willing to put off a contentious short-term disability program until it's reviewed by a committee of union and management representatives.

But with little more than four hours to go until the deadline, the union sent out a statement saying it had "initiated strike activity" to push for a better deal.

"The purpose of our strike is to encourage CPC management to return to the bargaining table with a proposal that meets the needs of current and future postal workers," the union said.

Canada Post says it hopes to keep the union at the bargaining table, and a meeting between the two parties is scheduled for Friday. But the corporation's proposed changes to sick leave are a key sticking point for the union.

In an earlier statement, the Canadian Union of Postal Workers said the Winnipeg local will be the first to walk off the job.

"Winnipeg has been chosen as the location for the first strike activity because it was the first city to be impacted by Canada Post's modernization program," the union said, explaining that the initial 24-hour job action is intended to "encourage Canada Post Corporation to abandon their proposals for significant concessions and instead negotiate solutions."

In an interview with CTV News Channel, the national director of the CUPW in the prairie region said that means both mail sorting and delivery will grind to a halt.

While letters and packages will not be delivered, Gord Fischer said pensioners and others expecting social assistance cheques needn't worry.

Thanks to an agreement between the union and Canada Post, he said volunteers will be working on set days throughout whatever strike action develops to ensure those types of items reach their destination.

"We don't want to harm the pensioners or the people who are receiving social economic (sic) cheques," Fischer said.

After the first walkout in Winnipeg, the union says it will announce where the strike action will go next.

On Monday, the union provided mandatory 72-hour notice prior to strike action. But negotiations continued.

Several meetings on Wednesday proved fruitless, including a sit-down between the union met and Canada Post President Deepak Chopra, as well as the Crown corporation's chief negotiator, Mark MacDonell.

After those talks ended, the union, Chopra and MacDonell attended a separate meeting with Labour Minister Lisa Raitt, who urged the two sides to resume negotiations.

But the two sides ended those talks still far apart.

Canada Post says mail volumes have dropped more than 17 per cent since 2006, taking revenues down accordingly, and it must address labour costs as a result.

"We have challenges at Canada Post when you consider how much the Internet is eating into our business and the amount of mail in the system," Hamilton said.

To that end, the corporation has proposed a starting wage of $18 for new hires, as well as new pension rules.

Key sticking points cited by the union include health and safety problems stemming from new work methods and equipment, as well as sick leave rules for employees.

Fischer said that the contract proposed by the corporation would change how postal workers handle the mail and would make it "less safe."

"We've done an ergonomic study and we're saying we already have letter carriers with a high injury rate -- we need something that's actually going to make the job safer," he said.

However, the union's proposals come with a price tag that's unacceptable for Canada Post, Hamilton said.

The union has been in a legal strike position since May, when its members voted 95 per cent in favour. Negotiations around a new contract have been ongoing for seven months.

The last time postal workers went on strike was 1997, when their two week job action ended with federal back-to-work legislation.

With files from The Canadian Press