The future of General Motors Corp.’s Antwerp, Belgium, plant is in doubt, following the auto maker’s decision to build its next-generation Astra small car at four other European plants.

GM says it will spend €3.1 billion ($4.2 billion) to produce the next Astra beginning in early 2010 at facilities in Ellesmere Port, U.K.; Bochum, Germany; Trollhattan, Sweden; and Gliwice, Poland. The auto maker says it will achieve a 30% productivity improvement with the launch of the new model.

The Antwerp plant is not on that list, and GM says the future of the facility is unclear beyond 2010.

“We are not talking about a plant closure,” the auto maker says in a statement. “However, we have to achieve the necessary improvements. No decision has yet been taken on future production, but we will work on options of assembly operations at fair volumes together with the European Employee Forum (the auto maker’s labor panel).”

The Antwerp plant currently employs 4,500 workers and produces the 3- and 5-door and station wagon and convertible versions of the current-generation Astra. It also is the source for the Saturn Astra that will be imported into the U.S. beginning later this year.

The near-term news is even grimmer for Antwerp, which will see production slashed this year as a shift is cut and 1,400 jobs eliminated.

“I know that today’s announcements will be very difficult for our workforce in Antwerp,” GM Europe President Carl-Peter Forster says in a statement. “The automotive business remains very challenging and GM Europe must continue to focus on increasing productivity and efficiency in order to compete effectively.”

Forster says GM Europe plans to produce 750,000 compact cars annually, up from 530,000.

“Even with an increase, adjusting capacity to meet demand remains a challenge for GM as it is for other auto makers worldwide,” he says.