GM also announced that 8,300 jobs would be cut across Europe and that capacity on the continent would be reduced by a fifth.
Opel said that it expected car sales in Western Europe to shrink by 1.5 million in 2010, and be around 4 million lower than in 2007.
The market would not return to this peak level any time soon, if ever, the company said, adding that it planned to close the Antwerp plant "in the course of the year 2010".
"In order to keep Opel going and have a sustainable future we have to take some of these types of decisions. We're not unique in this at all," Reilly said.
Jan Van Aggelpoel, a member of the ACV union in Antwerp, said: "We remain open until June 2010. Afterwards we will have to see what the possibilities might be. Otherwise, it's the end."
Opel workers crowded into a room at the plant on Thursday morning to learn their fate, emerging with a mixture of fury and despair.
|Reilly said 'we have to take a plant out and unfortunately it's Antwerp' [AFP]
Ahmed Teghmas, a production worker for 18 years, said he and fellow workers were angry, saying management had only told unions of a possible closure.
"They are playing a game in order to keep production going," he said.
Eddy Quirynen, another employee, said: "I'm 55 years old. What is possible for me now?"
The regional government of Flanders, where the northern port city of Antwerp is located, was to hold a meeting about Opel later on Thursday.
It had promised $710m in loan guarantees and a sale and leaseback deal if Opel Antwerp's future was secured.
Unions have repeatedly argued that closing Antwerp and letting go of staff there would be costly and drain liquidity that could otherwise be used to invest in new models.
Reilly said on Thursday he could not predict the cost of closing Antwerp without talking to labour representatives and added he did not anticipate industrial action as a result of the decision.
Opel's European works council said it would not make any wage concessions, following the decision.
GM, which lost $88bn between 2005 and 2008, went bankrupt last year and was bailed out by the US government in return for a pledge to restructure.
Reilly confirmed that about 4,000 of the total job cuts on the continent would be made in Germany.