OSHAWA, Ontario — Mayor John Henry was enjoying a quiet Sunday watching the Grey Cup, the championship game of the Canadian Football League, when his phone began ringing with calls from reporters wondering whether he had heard the rumors that General Motors was closing his city’s assembly plant.

He had not. In fact, the call from GM confirming the whisperings came just five minutes before it announced Monday that it was pulling the plug on five plants in North America, including the one in Oshawa, in an attempt to trim costs and shift its focus to electric and autonomous vehicles.

“There was no heads-up,” said Henry, whose grandfather, father and brothers have worked at the Oshawa plant, an imposing structure in the city of nearly 160,000 people about 40 miles east of Toronto.

Nadine Baldry, who has worked at the Oshawa plant for 16 years, felt blindsided, too. She told the Canadian Broadcasting Corp. that the news, which she first learned about on Facebook, left her “in shock.”

The number of people employed by the Oshawa plant has fallen dramatically from 23,000 during its heyday in the 1980s to roughly 2,800 now, but the news that it would no longer be manufacturing products as of December 2019 rocked the city — once considered the automotive capital of Canada — and prompted rebukes by lawmakers from all levels of government.

It also punctured much of the optimism felt after the renegotiation of the North American Free Trade Agreement. Just weeks ago in a Vancouver Sun op-ed, Canadian Foreign Minister Chrystia Freeland wrote that the new deal would ensure that “the car industry now has stability and room to grow and thrive.”

Canadian Prime Minister Justin Trudeau expressed his “deep disappointment” and said his government would do what it could to get workers “back on their feet.” Ontario’s provincial government pledged to extend unemployment insurance eligibility an additional five weeks for those affected.

Jerry Dias, the president of Unifor, the union representing Canada’s automotive workers, met with Trudeau on Tuesday. In a news conference after the meeting, Dias, who advised the Canadian government during its renegotiations of NAFTA, said that GM had shown Canada and the United States its “middle finger.”

He told Oshawa’s GM workers, who walked out in protest in response to the news, that “they are not closing our damn plant without one hell of a fight.”

When GM filed for bankruptcy in 2009 during the global economic downturn, the Canadian and Ontario governments provided more than $10 billion in bailout funding to help rescue the American auto giant.

But it seems unlikely that a similar bailout package is on the table this time. Navdeep Bains, Canada’s minister of economic development and innovation, said that while the Canadian government is “willing to engage and work with” GM, the company’s leaders had “been clear about their position.”

Dennis DesRosiers, whose company DesRosiers Automotive Consultants is a prominent automotive industry analyst, said that GM’s production in Oshawa has declined nearly 85 percent over the past 15 years.

“The writing has been on the wall for quite some time, so it was a matter of when not whether they would make this move,” he said in an email.

Still, it’s hard to overstate the degree to which GM’s Oshawa plant is stitched into the fabric and history of the city. Its hockey team, the Oshawa Generals, is named after the company. The team even felt compelled to issue a statement promising to “look for ways to support our GM worker fans, ticket holders and their families.”

The company first set up shop in Oshawa a century ago, in 1918, when it bought a car manufacturing business run by the McLaughlin family, whose name appears on libraries, art galleries and street names in Oshawa. The family, which immigrated to Canada from Ireland, was the largest manufacturer of horse-drawn sleighs and carriages in the British Empire at the end of the 19th century before it switched to producing cars.

Today, the plant manufactures the Cadillac XTS and Chevrolet Impala and assembles GMC Sierra and Chevrolet Silverado trucks.

Nancy Shaw, chief executive of the Greater Oshawa Chamber of Commerce, said the effects of the plant’s closure are likely to be felt elsewhere in the city. Many companies in the area sell auto parts to the plant, and the plant’s workers are constant fixtures at local businesses.

At Bridgette’s Family Restaurant, which has for years served breakfast and lunch to GM workers, owner Bridgette Legere said the news was “rough for everybody,” adding that it was not uncommon for multiple generations of the same family to work at the plant.

“For us, it truly is the end of an era,” said Shaw, whose mother once sewed seat belts for GM vehicles. “It will leave a hole for us.”