The federal government has widened an investigation into charges that many of General Motors Corp.'s A-body cars are subject to rear-brake lock-up, a condition that could cause loss of driver control.

The National Highway Traffic Safety Administration (NHTSA) this week upgraded the nearly year-long probe to "formal investigation" status -- a step that could lead to a final determination of defect and the forced recall of all of the affected automobiles.

The cars in question include Chevrolet Celebrity, Pontiac 6000, Buick Century and Oldsmobile Cutlass Ciera models manufactured for model years 1982 through 1984 -- about 1.9 million cars, according to GM's count. All are front-wheel-drive, mid-sized vehicles.

The NHTSA said it is aware of 688 owner complaints -- including reports of 114 accidents and 44 injuries -- involving the A cars.

"The decision to open a formal investigation is based on information gathered during the NHTSA engineering analysis begun in August 1984 on GM's A and J cars," the agency said. "The agency has decided to exclude the J cars from the formal investigation because almost all complaints to the agency involve the A models."

The NHTSA in January urged GM to recall the A cars voluntarily. But GM rejected the request on the grounds that the cars had no defects.

GM, in response to the NHTSA's decision to open a formal investigation, reiterated its belief that the A models have no braking problems.

"The action by NHTSA is based on a complaint rate that is extremely small in relation to the total volume of A cars covered by the NHTSA investigation," the auto maker said in an official statement. "We have evaluated the A-car brake system and are confident that there is no defect," the company said.

GM is in U.S. District Court in Washington facing similar charges of braking defects in its now-discontinued X-car line. The NHTSA, through the Department of Justice, has filed a $4 million lawsuit against the auto maker and is asking the court to force GM to recall 1.1 million 1980 X models suspected of having faulty brakes.

In another legal matter related to the X-car trial, the NHTSA is accusing GM of knowingly marketing the early X cars with defects and of lying to the government in an attempt to conceal its actions.

GM has denied all of the charges in the X-car trial, which began March 13, 1984. Evidentiary proceedings in the first part of the case -- which concerns the request for forced recall -- ended May 23. Final arguments are expected to begin later this year.