British Airways said yesterday it will place an order with Boeing Co. for wide-body jets potentially worth $6.9 billion, among the largest orders ever placed with the U.S. aerospace giant.

British Air also said it had negotiated the right to buy a larger airplane that Boeing hasn't even decided to develop.

Despite the large new order for 747-400 aircraft, which will add to its already bulging $91 billion backlog of orders for 1,700 new planes, Boeing said yesterday it is reducing its work force by an additional 2,700 in the second half of 1990, bringing the total job reduction for the year to about 5,600.

Boeing spokesman Paul Binder explained that reductions in the commercial jetliner work force are needed because the 747-400 jumbo jet is past its startup phase, requiring fewer workers to operate the assembly lines. In addition, the company is anticipating slower orders for its military aircraft. Boeing is a major subcontractor to Northrop Corp. for the B-2, which now faces growing opposition from members of Congress hoping to make major cuts in the Defense Department procurement budget.

Dale Shellhorn, Boeing's B-2 program manager, said last month that as many as 1,500 workers would be laid off or reassigned because of expected congressional cuts in the bomber program.

Binder said the company hopes to accomplish most of the reduction through attrition. All layoffs will occur in the state of Washington.

The sale to British Airways involves firm orders for 21 new 747-400s, costing $4.1 billion, and options on 12 more aircraft, valued at $2.8 billion, according to an airline spokesman. Deliveries of the new order are to start in 1992 and to run until 1999.

Rolls-Royce PLC won the $1.07 billion contract for the 747 engines, said Rolls-Royce spokesman Dick Dalton in Reston.

Meanwhile, UAL Corp., the parent of United Airlines, had been expected to announce within the next few weeks the largest commercial jet purchase ever, possibly to include Boeing's proposed new 777 wide-body. However, the employees' group seeking to buy UAL and the corporation's board of directors announced this week that talks with aircraft manufacturers had been suspended until the employee group could secure loans for a buyout. UAL directors said the aircraft negotiations are on hold until Aug. 9, the deadline for the employees to secure financing.