The dismal U.S. auto market took a turn for the worse in October, with sales plunging by about a third as the financial crisis and tightening credit kept buyers away from showrooms.

Auto makers sold 838,186 cars and light trucks last month, according to a tally by Autodata Corp. General Motors Corp. said it was the worst October in 25 years. When adjusted for increases in the U.S. population, last month was "the worst month in the post-World War II era," Michael DiGiovanni, GM's top sales analyst, said in a conference call. "This is clearly a severe, severe recession."

Auto executives warned that the market could deteriorate further, raising the question as to when the auto industry -- a key driver of the U.S. economy -- will hit bottom.

The modest decline in U.S. economic output in the third quarter "is not likely to be the worst we will see in this cycle," Ford Motor Co. economist Emily Kolinski Morris said in a company conference call.

A closely watched industry figure, the seasonally adjusted annualized selling rate, was 10.6 million vehicles, compared with 16 million a year earlier, according to Autodata.