The union representing security screeners at Toronto's Pearson airport says 68 jobs are being lost and another 231 are being cut back to part-time.
The Canadian Airport Workers Union says that will affect how long it takes to get through security and how safe it is on planes leaving the airport.
The union is warning Transport Canada that its workers can barely manage airport traffic with their current workforce.
"You're going to see lineups like we see down in the [United] States," union lawyer Denis Ellickson said.
The screeners work for the security firm Garda, which is contracted by the federal government.
The union says the Crown corporation that looks after airport security ordered the cutbacks, but the Canadian Air Transport Security Authority says the contractors control the workforce.
"We don't ask our providers to hire or lay off employees," said Mathieu Larocque, spokesman for the authority. "It is their decision."
Larocque said any staffing changes are due to bids that contractors made during the last request for proposals.
He said the staffing changes would not affect security and were meant to improve customer service.
"The resources that are needed to achieve these results are decided by the contractors themselves," he said. "They propose the business model and they manage our workforce. How our providers are staffing their shifts and their lines is entirely up to them."