NEW YORK - Shares of General Motors Corp. lost nearly one-third of their value Thursday, plunging to their lowest level since July 1950 after Standard & Poor's said the automaker's credit could fall further into junk status due to the "rapidly weakening state" of the global automotive market.

GM shares plummeted $2.15, or 31.1 percent, to close at $4.76 after falling as low as $4.65. That low marked the automaker's lowest trade since March 15, 1950, according to the Center for Research in Security Prices at the University of Chicago. At that time, the Korean War was three months away from beginning, and gasoline cost 27 cents a gallon.

Thursday marked the sixth straight day of losses for GM. The automaker's shares are down 50 percent from their close of $9.45 at the end of last month.

Brett Hoselton, an analyst who follows GM stock for KeyBanc Capital Markets, said a number of factors could be behind Thursday's drop, including the decline in banking stocks.

"Obviously, GM and Ford, they're closely tied to automotive financing," Hoselton said. "If you can't finance cars, you can't sell cars."

In addition, the three-week ban on short selling some stocks - including GM's - expired late Wednesday. Short selling involves borrowing a company's shares, selling them, and then buying them back when the stock falls and returning them to the lender. The practice allows investors to profit from the decline in a stock's value.

Dave Healy, analyst for Burnham Securities, said it's possible that the expiration of the short-sell ban hurt Ford and GM, though there is no way to know for sure.

"Both stocks have been favorites of the short-sellers" he said. "These are volatile stocks. They go down 10 percent when the market goes down 5 percent, and vice versa."

Still, the automobile industry has been bombarded by a spate of gloomy news. In the closing minutes of trading Thursday, Standard & Poor's Rating Services placed GM and its finance arm, GMAC, on "CreditWatch Negative," meaning a downgrade of its "B-" long-term corporate credit ratings could be forthcoming.

S&P did the same to Ford Motor Co., helping send the Dearborn, Mich., company's stock down 58 cents, or 21.8 percent, Thursday to close at $2.08. It had fallen as low as $2.03 earlier in the session, it's lowest price since June 1, 1983.

Analysts have voiced concerns that the ongoing slump in U.S. vehicle sales could last longer than they previously expected and could spread to other parts of the world, particularly Europe.