A substantial number of workers at a General Electric Co. defense plant here will be laid off because of a drop in orders from the Pentagon, the company announced Thursday. Arthur Glenn, vice president of GE's Defense Systems Division, refused to say exactly how many workers would be laid off or when. However, GE officials informed the state it would lay off about 900 workers over the next 18 months because of a decrease in defense-related orders, according to a state economic affairs official. The first phase of the layoffs is scheduled for July, according to the official, who asked not to be identified. The unexpected announcement by the company came only three months after GE promised stable employment at its Pittsfield businesses this year. The company had laid off 1,000 workers last year when it closed its transformer plant. The ordnance division, which employs 4,700 people, makes guidance systems for Trident submarine-launched nuclear missiles, components of the Bradley fighting machine, which carries infantrymen, various high-speed guns and other military products. ``The market we serve we thought, up to a year ago, to be at least in a sustaining basis and somewhat of a growth basis,'' Glenn said at a news conference. He blamed the sudden turnaround on government cutbacks in ordnance contracts. ``As pressure gets tough on defense budgets, government customers have to make difficult decisions,'' Glenn said. He said a government contract the company recently lost to Northrup Industries had little to do with the layoffs. ``We had really planned our business based on not winning that contract because there was so much controvery over whether the Air Force wished to use our technology,'' Glenn said. ``Certainly if that contract had come, I would not have had to take as severe an action as we're going to have to take without it.'' Glenn also said a decrease in orders would mean a slowdown in Bradley fighting machine production, from 550 yearly to about 350. He said he told the company's 500 management employees Thursday morning about the impending layoffs. Neither managers nor union officials could be immediately reached for comment. GE now employs about 6,700 people in the ordnance and plastics plants in the city, the number it had employeed in 1983, before the company began expanding the ordnance systems division with 1,400 new employees by late 1986. Workers at the company were dealt a major blow in the fall of 1986 when GE announced it would close its transformer division in Pittsfield. GE announced in August 1987 it was reorganizing the ordnance division to eliminate the engineering and manufacturing department and replace it with a Division Effectiveness Operation to improve efficiency. ``The steps we are taking are usually taken by businesses in trouble and we are not in trouble,'' division Vice President Nicholas Boraski said at the time.