General Motors Corp. yesterday slashed its stock dividend by nearly half, announced a new round of white-collar layoffs and said it will scrap executive bonuses for 1990 in a dramatic recognition that the U.S. auto industry is suffering its weakest period in nearly a decade.

The company also forewarned shareholders that it may report a heavy fourth-quarter loss, which some analysts have estimated at $1.4 billion. That would make it one of the largest three-month operating losses ever recorded by an American corporation.

The news comes amid continued bleak sales reports, not just at GM, but at Ford Motor Co., Chrysler Corp. and most of the major Japanese automakers with plants in the United States. Production levels at some plants have been cut in half as automakers feel the effects of consumers delaying new-car purchases until they feel more settled about the economy.

About one-quarter of the nation's 475,000 hourly auto workers are on layoff, the largest number since the 1981-82 recession.

GM's dividend reduction, from 75 cents to 40 cents, is its first since April 1980. With nearly 2 million stockholders nationwide, the automaker's shares are among the most widely held of any company in the country.

The cutbacks in white-collar staff will eliminate 15,000 administrative jobs from GM's North American work force by 1993. Some 6,000 of those will disappear this year. Since 1986, GM has slashed some 40,000 office jobs from its payroll -- most of them in the United States -- bringing its white-collar employment in the United States to 99,000.

The dividend cut, layoffs and a $500 million cut in spending for plants and equipment are aggressive moves taken in light of current economic conditions brought on in part by the turmoil in the Middle East, said GM Chairman Robert C. Stempel.

"Clearly, in this highly uncertain economic environment, we must conserve cash and reduce costs further in order to provide GM with sufficient financial resources to maintain our important forward product-development programs which are so vital to our market share and long-term profitability," Stempel said.

Several auto industry analysts agreed that such cuts were necessary given the outlook.

"What GM is doing is a sign of the times," said Christopher Cedergren, an analyst with J.D. Power and Associates, a California-based auto-market research firm. "This kind of bad news most likely will continue in 1991, not only for the Big Three {GM, Ford and Chrysler}, but also for the Japanese and the Europeans. It is not going to be a good year."

Analyst Gurudutt Baliga, with McDonald & Co. Securities in Cleveland, agreed that the outlook is likely to grow more dismal. "The company is expecting a substantial loss for the fourth quarter of 1990 and for the whole year," said Baliga. "They're also looking at a weak auto market" that shows little sign of improving in 1991.

Under such circumstances, "It is a very favorable thing that GM is doing," Baliga said. "It shows that the management of the company is acting responsibly, and in the long term, that will help GM."

An estimated 14.1 million cars and trucks were sold in the United States last year, 5 percent less than the 14.8 million sold in 1989. Many analysts are predicting that car and truck sales will total 13.5 million or slightly less in 1991 -- putting enormous pressure on the profitability of all car companies doing business in the United States.

For GM, last year was not a good year, either. The company reported a third-quarter loss of $2 billion. Most of that, however, stemmed from $2.1 billion the company set aside to cover the cost of a series of plant closings. Without the charge for shuttered plants, GM would have earned a third-quarter profit of $109 million.

Stempel said yesterday that GM expects a "substantial" fourth-quarter 1990 loss, but declined to define "substantial."

For all of 1990, analyst Baliga said he is estimating that GM will post a loss of $2.1 billion, compared with earnings of $4.2 billion in 1989.

"Hopefully, this situation will be short-lived, but in the interim, we can't afford to be tentative," Stempel said of the conditions prompting GM's latest cutbacks. "While we expect the U.S. economy to start a gradual recovery at some point during 1991, we cannot rely on an economic turnaround alone to secure an adequate financial return for our stockholders over the longer term."