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A blockade at the busiest route linking Canada to the United States is further snarling global supply chains, leading to production stoppages and other difficulties for automakers and other manufacturers with dwindling inventories.

Automakers, who have already been suffering from a global shortage of semiconductors needed to power their cars, are being particularly affected by the partial shutdown of the Ambassador Bridge, which links Detroit, Mich., with Windsor, Ontario, and accounts for roughly a quarter of trade between the two countries.

Ford Motor Company said it had shut down two Canadian plants and reduced production at another as of Wednesday afternoon. Toyota Motor Corporation and Honda Motor Company would likely be closing some production lines later on Wednesday because of border closures, said David Adams, the president at Global Automakers of Canada, which represents both companies.

Mr. Adams declined to specify which plants would be affected, but said Toyota and Honda together had six different facilities in the vicinity that depend on shipments across the bridge. Thousands of trucks cross the bridge daily, ferrying auto parts between major vehicle plants.

“It’s pivotal, certainly, to the automotive industry,” Mr. Adams said.

Both Toyota and Honda operate on just-in-time supply chains that deliver parts to factories as they are needed, giving them enough inventory to operate for about two days without new deliveries before production lines need to slow, he said. Protests over Canadian vaccine mandates began partially blocking off traffic over the Ambassador Bridge on Monday night.

In a statement Wednesday afternoon, Shane Wark, the assistant to the national president at Unifor, which represents Canadian autoworkers, said the protests at the border continued to disrupt work at Unifor-represented auto plants, resulting in short-term layoffs at a Ford engine plant in Windsor, a Ford assembly plant in Oakville, near Toronto, and the Windsor assembly plant of Stellantis, the company that owns Fiat Chrysler.

“The situation is fluid, and changing by the hour,” he said.

“These blockades are creating added hardship on Unifor members and their families in the auto sector, following two years of extraordinary production and supply chain disruptions, and must come to an end immediately,” he added.

In a statement, a Ford spokesperson, Said Deep, said the interruption hurt “customers, autoworkers, suppliers, communities and companies on both sides of the border that are already two years into parts shortages resulting from the global semiconductor issue, Covid and more.”

“While we continue to ship our current engine inventory to support our U.S. plants, we are running our plants at a reduced schedule today in Oakville, and our Windsor engine plant is down,” Mr. Deep added.

In a briefing Wednesday, Jen Psaki, the White House press secretary, said that the blockade posed a risk to auto industry supply chains, and that the administration was also tracking potential disruptions to agricultural exports from Michigan into Canada.

Biden administration officials were in close contact with customs agents, officials in Canada and Michigan, and industry stakeholders to speed traffic and monitor the impact on supply chains, she said.

“We’re working to ensure there’s movement,” Ms. Psaki said.

“I think it’s important for everyone in Canada and the United States to understand what the impact of this blockage is — potential impact — on workers, on the supply chain, and that is where we are most focused,” she added.

Companies have been looking at alternative crossing routes, like the Blue Water Bridge, which links Port Huron, Mich., and Sarnia, Ontario, to ferry their supplies. But a new blockade on a route to that bridge, as well as a surge of diverted cars and trucks, slowed traffic there as well.

Matt Blunt, the president of the American Automotive Policy Council, which represents Chrysler, Ford Motor Company and General Motors, said the protests had already resulted in lost production.

Mr. Blunt said his group had encouraged the Biden administration to reach out to Canadian counterparts. The Biden administration and customs officials appeared to be doing everything they could from the U.S. side to facilitate as much commerce as possible, he said.

“At some point, if there’s no entry or exit from Canada, there’s not much that can be done,” Mr. Blunt said. “Every day that this shutdown or slowdown persists, it’s going to have an impact on U.S. and Canadian production.”