"None of the potential investors (were) able to come forward with a sustainable business concept for the plant," Dow Jones Newswires quoted an Opel statement as saying.
"The ... search (for) an industrial investor interested in continuing operations has come to an end," Opel added.
The Belgian plant employs around 2,600 people and was founded in 1924.
An Opel spokeswoman told AFP that at the end of the year, "production will cease and the plant will be closed," but added that the automaker "is still open to talks" on the site's future.
Belgian trade union CSC said Monday that the decision to shut the factory "constitutes the coup de grace for workers and suppliers of Opel Antwerp."
Opel chief executive Nick Reilly said last week that Opel might look for another investor for the plant, perhaps in the logistics sector.
Although the company expected to make a profit in 2012, it would probably report "a significant loss" this year and be hit by further restructuring charges in 2011, Reilly said.
GM wants to cut its European auto capacity by 20 percent and eliminate around 8,000 jobs so as to focus on Opel and its British sister brand Vauxhall.
The US parent company decided not to pursue European state aid for the restructuring after talks with Germany failed following an aborted attempt to sell Opel to Canadian auto parts maker Magna.