General Motors plans to announce as early as Monday it will cancel six car models, including the plug-in hybrid Chevrolet Volt, Reuters reported Monday, citing an unnamed source.

GM is reportedly planning to stop allocating vehicles to three assembly plants. Canadian union Unifor said GM had informed them of their plans to stop allocating products to the Oshawa Vehicle Assembly plant in in Ontario, Canada in December 2019.

The largest U.S. automaker also plans to cut production at plants in Michigan and Ohio, Reuters reported.

GM did not immediately respond to a request for comment to CNBC on Monday.

GM has been cutting jobs to deal with plunging car sales in North America. The company has internally debated for months how to address shrinking car demand, a person briefed on the matter told Reuters, and the issue is certain to re-emerge when GM holds contract talks next year with the United Auto Workers union.

GM is expected to announce as soon as this week some involuntary salaried layoffs after it did not get as many volunteers to accept buyouts as hoped, the person briefed on the matter said.

GM said on Oct. 31 that about 18,000 of its 50,000 salaried employees in North America are eligible for the buyouts.

The stepped up cost-cutting and restructuring at GM comes as many industry executives and analysts predict that overall vehicle sales in the United States will decline further in 2019 and 2020.

At the same time, China, the world's largest auto market and GM's biggest market by vehicle sales, has decelerated sharply in the past few months.

Analysts have said GM has too many North American plants building slow-selling sedans.

GM shares are down 12 percent for the year, and GM Chief Executive Mary Barra, in a message to employees last month, cited the stagnant share price as a reason for tougher restructuring measures.

She said the automaker had negative cash flow for the first nine months of the year and it needed to cut costs.