Transport Minister Steven Fletcher and Labour Minister Lisa Raitt are demanding security personnel cease "unlawful strike activities" at Toronto's Pearson International Airport that left travellers facing major delays and missed flights Thursday and Friday.

Security services provider Garda, contracted by the Canadian Airport Transit Security Authority (CATSA), says its workers are intentionally slowing down the screening process to protest changes to their work schedules.

And with many hoping to travel for the Thanksgiving long weekend, airlines are concerned that delays at Pearson can affect service at other airports.

"If the flight is late leaving Toronto, it's going to be late arriving in Vancouver, and then late leaving Vancouver and late arriving in Winnipeg and so on," Air Canada spokesperson Peter Fitzpatrick told CTVNews.ca in a phone interview Friday afternoon.

CATSA spokesperson Mathieu Larocque told CTVNews.ca that employees had been ordered by the Canada Industrial Relations Board to resume normal working order, but "we're certainly not seeing that right now."

"It's true it's a busy weekend, but we shouldn't see the lineups that long just because of the heavier traffic of Thanksgiving. The delays are clearly being caused by this work action," he said.

In a joint statement released just before 6 p.m. Friday, Fletcher and Raitt said they expect the union to "desist from participating in any further unlawful strike activities, and to resume service levels" at Pearson.

Raitt has appointed a federal mediator to assist in resolving the dispute.

"We are also pursuing alternative measures to ensure that passengers and baggage can be properly screened," said Minister Fletcher in the statement. "The government will pursue any and all additional actions to ensure Canadians spend this Thanksgiving weekend with their families, not in line at the airport."

In their own statement, released shortly after 3 p.m. Friday, Garda said they "expect the screening process to return to normal within the next hours and will continue to take the necessary steps to efficiently perform the screening process without compromising the security of the travelling public."

Traveller Gavin Rooke goes back and forth between Toronto and Vancouver for work on a weekly basis. Lineups for his 5:30 p.m. flight to Vancouver took hours Friday, compared to the usual 10-15 minutes.

"Once you got to the front, the line was moving and the actual processing of people didn't seem much slower than normal, so I don't know if maybe earlier it was slow and then it just backlogged." He told CTVNews.ca in a phone interview from the terminal.

Air Canada has had reports of two hour waits – and many customers have missed their flights altogether. The airline is expecting 100,000 customers through its system today – 30,000 of which are expected to travel through Pearson's Terminal 1.

Air Transat spokesperson Debbie Cabana told CTVNews.ca they're also experiencing delays at Terminal 3, but had not received any formal customer complaints. They're advising customers to check all luggage to save time at security.

Airport spokesperson Scott Armstrong said work-to-rule action in the airport's pre-boarding area, where travellers pass through X-Ray machines, is causing the massive backlog.

"There are long lines for security screening in Terminal 1," Armstrong told CTVNews.ca. "Right now, mainly domestic flights and flights to the U.S. seem to be affected."

Many passengers stuck in security line-ups took to Twitter Thursday morning, reporting wait times as long as five hours.

"Most epic mission to catch a plane ever! Folks, if you're flying out of [Pearson] today give yourself at least 3 hours. Seriously!!," wrote one traveller in a message posted to Twitter.

About 200 full-time security screeners who worked at Pearson were laid off in May of this year after their employer said it would be cutting back hours, months after several part-time workers were hired.

Pearson Airport representatives are encouraging travellers to check their flight status before arriving at the airport. Passengers can check their wait times here.