Justice Department officials moved yesterday to head off a possible beer industry merger.

Attorney General Griffin Bell said the antitrust division would file a suit against Pabst Brewing Co. and Carling National Breweries, if the two firms go ahead with plans for Pabst to acquire Carling National.

Pabst, based in Milwaukee, is the country's fourth-largest brewer but its sales and profits have been declining and its share of the domestic market has shrunk because of intense competition from the three largest firms - Anheuser-Busch, Philip Morris' Miller Brewing and Joseph Schiltz & Co.

Last April, Pabst President Frank DeGuire said his firm was having "ongoing discussions" about a takeover of Carling National, based in Baltimore. Carling O'Keefe Ltd., of Canada, sold its control of the Baltimore firm late last year to R&R Holdings Ltd., an associated company of Britain's Rothmans Group.

Carling National itself represented a combination of Carling's U.S. operations and National, a former regional brewery in Baltimore. Except for a small profit in the fiscal year ended March 31, 1977, Carling National has been unprofitable since 1971.

Like other firms outside the circle of the two dominant brewing companies - Busch and Miller - Pabst and Carling National have been scrambling to retain a share of the increasingly concentrated U.S. industry.

American beer consumption has increased 25 percent in the past decade but the number of brewing firms has declined to less than 50 from 118 in 1965. The top five brewers now account for 70 percent of sales and that figure is expected to be 80 percent by 1981.

A brief Justice Department announcement said only that the government would contest a combination of the breweries as an alleged violation of the Clayton Act. Spokesmen for the companies declined to comment or could not be contacted last night.

If Pabst took over Carling National, the Wisconsin firm would solidify its hold on the No. 4 spot in the industry and put it in a position to challenge Schlitz. Pabst sold more than 15 million barrels last year (10 percent of the U.S. market) while Carling National, now 10th largest, sold less than 5 million barrels and accounted for 2.3 percent of the total market.