General Motors Corp. said it will reduce the work force at its Baltimore manufacturing plant from two shifts to one next summer, but will keep the plant operating through the fall of 2003.

More than 2,100 workers produce Chevy Astros and GMC Safari vans at the plant, one of GM's oldest and a cornerstone of Baltimore's manufacturing base for many years. Until yesterday's announcement, GM had not said whether the Baltimore plant would remain in operation after 2001.

The plant's prospects after 2003 will depend on demand for GM vehicles at that time, said Stan Flores, spokesman for GM Trucks. "It's too early to speculate about the future of the plant, but we have made no final decision on closing it," he said.

Maryland officials, who have been negotiating steadily with GM over the plant's fate, said they now have more time to persuade the automaker to keep the facility going. "This story isn't over now," said Richard C. Mike Lewin, Maryland's secretary of business and economic development. "We have four years to work on them, then it's up to market forces."

The shift reduction at the plant will eliminate about 1,200 jobs, GM said, although about 400 of the workers will be hired to staff a new engine plant that GM will open in White Marsh, in Baltimore County, in January 2001.