TORONTO -- Businesses are warning their customers about the possibility of a strike or lockout at Canada Post that could happen as early as this weekend.

A number of companies including TD Bank (TSX:TD), Virgin Mobile and American Express have told their customers not to expect statements in the mail in the event of a labour disruption.

Some are encouraging their customers to pay their bills online if they're not already doing so and avoid mailing in payments.

Unionized workers at Canada Post can legally go on strike or be locked out as of Saturday if an agreement isn't reached.

Canada Post and the Canadian Union of Postal Workers (CUPW) have been in negotiations since December for its 50,000 delivery and plant employees.

The Crown corporation says it tabled new contracts last Saturday, but have yet to hear a response from the union, which must issue a 72-hour notice before going on strike.

On Tuesday, CUPW asked for a two-week extension to give more time for contract negotiations. Canada Post denied the request.

"We're seeing the impact that the uncertainty is causing in our facilities," said Jon Hamilton, a spokesman for the postal service.

"The amount of mail, the amount of advertising mail, the number of parcels are already declining. Many of our customers have already put contingency plans in place months ago and have been moving away from Canada Post."

Canada Post says the main contentious issues relate to changes to pension plans for incoming workers and the hiring of temporary and part-time delivery employees for evenings and weekends.

Hamilton said a work stoppage will result in lost business in an industry that it is already feeling the squeeze from other delivery services and the growth of online payments.

"We understand our customers have businesses to run, and we've been trying to give them as much advance notice as one could," he said.

"Obviously we're hoping that we can get a deal and continue to operate, but when customers are looking for that certainty and we can't provide it, they need to make other plans."

Rivals Purolator and UPS said they are preparing to step in if a Canada Post labour disruption occurs.

Canada Post delivers approximately nine billion letters, parcels and flyers a year, serving nearly 15 million residential and one million business addresses.

The last labour disruption was in 2011, when Canada Post workers went on rotating strikes for 10 days.