The union representing more than 400 zoo staff is warning of a lockout after the zoo's management requested a "no board" report from the Ministry of Labour last week.
But the facility itself says it is just trying to work through the process of reaching a new deal.
Under Ontario labour law, once the "no board" report is issued, either party can trigger a work stoppage 17 days later, which means the zoo could be in a legal lockout position at 12:01 a.m. on April 26.
Canadian Union of Public Employees Local 1600 president Christine McKenzie says the union is "troubled and mystified" by the zoo's decision to request the report.
"We're really worried that they're going to be putting us in a lockout on the 26th, which would be of course, really damaging for the zoo with the upcoming pandas on May 18th opening up, and to the public and the workers obviously too," said McKenzie.
"We're willing to negotiate and we're very frustrated by the lack of response from the other side."
The union says cuts to bereavement leave, changes to sick pay and benefits, and job security are among the issues of contention.
Meanwhile, the zoo said requesting the no board report was the "next legal step available" in its efforts to negotiate a new agreement with the union.
"Although the parties have been meeting regularly at the bargaining table since February, conciliation with CUPE Local 1600 has not worked so far and the parties are at an impasse," said Jennifer Tracey, senior director of marketing, communications and partnerships at the zoo.
"We are continuing to bargain with CUPE Local 1600 and remain hopeful that a resolution is possible during this 17-day period."
CUPE Local 1600 represents a range of zoo workers including horticulturalists, maintenance staff, zookeepers, trades people, animal nutrition assistants and veterinary technicians.
The zoo's panda exhibit — featuring two giant pandas on loan from China — is to expected to draw huge crowds.
Five-year-old Er Shun and her prospective mate, four-year-old Da Mao, arrived in late March aboard the "Panda Express,'' a specially outfitted FedEx Express Canada plane branded with an image of a panda on its exterior. They have been in quarantine since their arrival.
The pair will be living in Canada for the next 10 years, splitting their time equally between the Toronto Zoo and the Calgary zoo.
Their arrival, the first panda visit in 24 years, marked the realization of a deal reached when Prime Minister Stephen Harper visited China just over a year ago.
If a work stoppage of any sort was to take place, the union representing zoo employees said the panda exhibit and the zoo's spring season could be affected.
"If we were to be locked out on the 26th, our workers wouldn't be able to do anything in terms of preparing the zoo for the opening of the pandas," said McKenzie.
"If it were to continue up to the middle of May then potentially the opening would be delayed, which would have a huge impact on our peak season and obviously on the many, many visitors that are excited to come and see them."