General Motors (NYSE: GM) is reportedly ending its relationship with the advertising agency responsible for Chevrolet’s ad campaigns, an account that's worth nearly $1 billion.

The split between GM and full-service ad agency Goodby Silverstein & Partners, which is expected to be announced as early as Wednesday, will result in a shift of the bulk of Chevrolet’s advertising business to McCann Worldgroup.

Rumors of the breakup surfaced on Monday after McCann CEO Harris Diamond, who was scheduled to moderate a morning political discussion panel at the 4As Transformation conference in New Orleans, canceled at the last minute due to a meeting with a client believed to be GM.

Multiple news outlets are reporting that final details have been hammered out in the last two days.

According to the Detroit Free Press, which cites a source familiar with the matter, there will not be any changes to GM’s marketing plans under the joint venture, called Commonwealth.

The decision to remove Goodby from the Commonwealth partnership puts an end to a deal that was spearheaded by former GM chief marketing officer Joel Ewanick.

Ewanick, who was fired last summer over a European soccer marketing deal, had established Commonwealth as a 50-50 venture between competitors Goodby and McCann and won publicity for enticing them to launch an office in downtown Detroit, DFP reports. The deal took place during the 2010 World Series when Goodby launched the "Chevy Runs Deep" ad campaign.

The massive changes come as Chevy gears up to launch three new products in 2013, including an all-new Silverado pickup, the redesigned Impala sedan and the much-hyped Chevrolet Corvette Stingray.

Kantor Media said that in 2012 GM spent almost $975 million advertising its Chevy brand in the U.S., Automotive News said. Cadillac spent $243 million on U.S. advertising last year.