The National Highway Traffic Safety Administration yesterday refused to reopen an investigation into an alleged car defect that is regarded by consumer groups as the most dangerous in the history of U.S. auto safety regulation.

The suspected flaw involves Ford Motor Co. cars and trucks manufactured for the 1970 through the 1979 model years. Many of the vehicles, about 20 million of which remain in operation, supposedly are equipped with automatic transmissions that can slip from park to reverse without warning.

NHTSA investigated the matter for three years, beginning in 1977. The agency ended the probe Dec. 30, 1980, after Ford agreed to send affected owners stickers advising them that, under certain conditions, their vehicles could move from park to reverse inadvertently.

Owners were asked to place the stickers on their dashboards as a reminder of the potential problem.

But consumer groups, chiefly the Washington-based Center For Auto Safety, said that the sticker campaign was useless. An estimated 104 people have died in Ford park-to-reverse accidents since the sticker campaign was initiated, the consumer groups said.

As a result, a coalition of consumer and safety organizations last March 6 petitioned NHTSA to reopen the matter.

NHTSA Administrator Diane K. Steed yesterday refused the request, saying: "I have decided to deny the petition because there are not sufficient reasons to expect further investigation of this matter to result in a determination that the vehicles in question contain the alleged defect."

Steed said the government's original decision to close the park-to-reverse investigation and to send warning labels was upheld by the U.S. District Court and by the U.S. Court of Appeals for the D.C. Circuit.

Those court decisions, coupled with the "absence of new information suggesting the presence of a safety-related defect and the uncertain prospects for the effectiveness of proposed mechanical remedies," support her decision not to reopen the investigation, Steed said.

Helen O. Petrauskas, Ford's vice president for environmental and safety affairs, agreed. "The NHTSA decision not to reopen the investigation . . . was the right thing to do because there is no defect in the design and manufacture" of the vehicles in question, Petrauskas said.

"The greatest need now is for all involved to urge the drivers of all makes of vehicles to use safe parking procedures," Petrauskas said.

Clarence Ditlow, CFAS director, said that Petrauskas' comments amounted to blaming the victims for Ford's mistakes. Of NHTSA's decision to stand by its 1980 ruling, he said: "This . . . is penned in the blood of hundreds killed while the Department of Transportation which has jurisdiction over NHTSA did nothing."

CFAS and other consumer groups involved in the dispute said yesterday they would file another federal suit in an attempt to overturn NHTSA's decision.