The government has sent a sternly worded letter to General Motors Corp. indicating that tens of thousands of the 1980 models of GM's popular X-body front-wheel-drive cars may have to be recalled to correct an alleged brake defect that could cause the rear wheels to lock.

The letter, sent last month by the National Highway Traffic Safety Administration and released yesterday, says that a previous recall of some of the cars apparently failed to correct the problem, which has allegedly caused crashes that have killed one person and severely injured several others. Many cars not recalled apparently have the same problem, NHTSA says.

The alleged defect might cause the rear wheels of early-1980-model X cars to lock when the brakes are applied, sending the car into a spin or skid, the government said.

It is unclear how many X cars may be subject to a recall. GM made more than 1 million X cars in the 1980 model year, under the Chevrolet Citation, Buick Skylark, Oldsmobile Omega and Pontiac Phoenix nameplates. The alleged defect was apparently corrected sometime during the 1980 model run, which began in April 1979, NHTSA officials said.

General Motors officials said they see no need to recall the cars but are cooperating with the government's investigation of the problem. A GM spokesman said, "This is a very difficult thing to assess. Braking is almost as subjective a subject to assess as handling."

Meanwhile, Rep. Timothy E. Wirth (D-Colo.) called for an investigation by the General Accounting Office of the NHTSA's handling of the X-car brake issue, charging that the agency has dragged its feet on the investigation of the alleged defect.

In its letter, the NHTSA says GM's voluntary recall of 47,000 of the cars 18 months ago "has not completely solved the rear brake lock-up problem." The NHTSA said it had received more than 60 complaints that the recall--which came shortly after the government began investigating the matter--had not cleared up the brake problem.

"In view of the seriousness of this situation, and the continuing receipt of complaints by this office, I urge General Motors to review this alleged safety-related defect," George L. Parker, director of the NHTSA's office of defects investigation, wrote the automaker.

"Please explain GM's position regarding the expansion of the GM recall to include all X-body vehicles which exhibit the lock-up problem. Your response should also include a discussion of additional corrective actions."

However, GM vice president for consumer relations and service James G. Vorhes said yesterday that since the recall, which involved adjustment of the brakes and replacement of a pressure valve, "we do not believe X-car braking systems present an unreasonable risk to safety."

Wirth, in a letter to GAO, charged that the agency may be "unnecessarily delaying" and "covering up the existence of very serious defects," Wirth wrote.

An NHTSA spokesman yesterday denied that the agency had delayed completion of the investigation--which began in July 1981--for any reason.

"We're trying to make a finding that either the recall worked or it didn't," he said. "I don't think anybody's playing games here."

He said the agency expected to release the results of the investigation within the next few weeks.

The government request to GM for addition information and investigation came after the NHTSA was petitioned by the Center for Auto Safety, a consumer-advocacy group, to expand the scope of the investigation to include later model X-body cars.

Clarence M. Ditlow III, director of the Center for Auto Safety, a consumer advocate group, said yesterday, "I guarantee you if you took a 1980 X car out right now, went down Massachusetts Avenue by the British Embassy and put the brakes on, you'd spin out," Ditlow said.

The alleged problem lies in a component of the cars' brake system called a proportioner valve, a device that apportions hydraulic pressure between the front and rear brakes.

The proportioner valves used in the early part of the 1980 model run were apparently set to give too much pressure to the rear brakes--41 percent of the total hydraulic pressure. During the model run, GM switched to a valve that gave 27 percent of the pressure to the rear brakes. GM installed these valves on the cars that were recalled, all of which had manual transmissions. GM apparently did not recall early-1980-model X cars with automatic transmissions and the 41 percent valves.

Government tests of the recalled and repaired cars, and of other early-model X cars, have shown that the problem has not been corrected, however, NHTSA says. Ditlow says that the recall should have included replacement of several other components of the braking system to solve the problem.