A consumer group yesterday blamed the deaths of six motorists on the failure of a government agency to crack down on Ford Motor Co. for alleged transmission problems in an estimated nine million Ford cars.

The Center for Auto Safety, in a letter to National Highway Traffic Safety Administrator Joan Claybrook, said it was worried about "protracted delays" in NHTSA's "investigation of defective automatic transmissions in Ford vehicles that jump [by themselves] from park into reverse."

NHTSA had indicated last summer that it was "close" to ordering Ford to recall the 1970-1978 automatic transmission cars because of consumer information that many had jumped out of park and into reverse.

Last August, however, NHTSA decided it needed more information before any action could be taken. It issued a "consumer advisory" asking Ford owners to contact the agency if they experience similar transmission problems.

"Since NHTSA released its consumer advisory reporting 23 deaths, 259 injuries and 777 accidents from this defect, the Center has received reports of 6 more deaths, 26 injuries and 41 accidents," center director Clarence Ditlow wrote Claybrook.

"The longer NHTSA delays," Ditlow added, "the more people will die from this defect."

Ford says there is no problem with the transmission. It had notified many car owners, asking them to report any incidents involving the transmission going out of control but a Ford spokeswoman said yesterday, "We have not verified slippage from park to reverse in any cars." But, the spokeswoman added, "We are still cooperating with the NHTSA in its investigation."

In an interview late yesterday, Claybrook said "there is no question that we are concerned about this problem. It is the highest-priority investigation going on at this agency. We realize that it involves a huge number of vehicles -- about 9 million -- and is such a serious matter."

But Claybrook said her agency has yet to determine whether the Center for Auto Safety's accident, injury and death figures are accurate.

Ford contends, NHTSA sources say, that the only reason NHTSA has received more transmission complaints about Fords than other cars was the fact that NHTSA asked Ford owners specifically to call if they had the problem.

Claybrook discounts that argument, however. "This is a unique and unusual kind of defect," she said. "If people had a General Motors car, and had this experience, they would call us."

The Ford investigation has been going on at NHTSA for about a year and a half, and the Center has been keeping track of transmission-related injuries for that long, including many vicitims who claimed that the transmissions jumped into gear when the car door was slammed and the car left running.

In yesterday's letter, Ditlow gave the names and dates of the transmission-related accidents which occurred since August and are alleged to have resulted in deaths.

Ditlow pointed out in the letter that the new accidents involve one 1979 car, and certain model transmissions that had not earlier been thought to have the alleged problems.