The U.S. Department of Commerce estimates that more than a third of the new cars sold in the United States in 1988 will be imports, compared with less than one-quarter last year, Automotive News reported yesterday.

The trade publication quoted a Commerce Department official as saying the decline in market share could put as many as 500,000 workers out of work -- 100,000 in the auto industry and 400,000 in the supplier industries.

Automotive News said the figures were released by the Commerce Department to a recent Washington meeting of Automotive Original Equipment Manufacturers Inc.

According to Commerce, new car sales in the United States will total 11.2 million in 1988, but 36 percent -- or 4.06 million -- of those cars will be imports. Last year imports accounted for 23.5 percent of the market share.

U.S. sales of cars built in North America will plunge to 6.37 million in 1988 from 7.82 million in 1984, Commerce said.

Robert Watkins, the department's deputy assistant secretary of automotive affairs, declined to give the manufacturers a specific number when asked what level of import penetration the Reagan administration would find acceptable in the U.S. auto industry.

Automotive News said Watkins did say, however, that the projected slide in the domestic auto market share was not "irreversible."

Using the Commerce Department projections for 1988, the estimated market shares would be 27 percent for Japan, 9 percent for other imports, 57 percent for domestic makers and 7 percent for Japanese cars built in the United States.

In comparison, the 1984 figures were 18 percent for Japan, 5 percent for other imports, 76 percent for domestic makers and 1 percent for Japanese cars built in this country.