Two powerful unions in the food industry, the Retail Clerks and Amalgamated Meat Cutters, announced yesterday they are close to what would be the largest merger in the history of the American labor movement.

The merger, expected sometime next year, would produce a combined union of more than 1.2 million members, the biggest in the 14-million-member AFL-CIO.

The merger underscores the dramatic shift of power within the labor movement from its historic industrial and craft union roots to the burgeoning ranks of government and service workers.

Less than a year ago, the largest AFL-CIO union was the United Steel-workers of America. It was leapfrogged by the American Federation of State, County and Municipal Employes (AFSCME) after a government employes' union merger in New York State this year. Now the Steelworkers will fall to No. 3 if the clerks and meatcutters merger goes through as expected.

The planned merger was announced jointly by presidents Bill Wynn of the Retail Clerks and Harry Poole of the meatcutters, who said the unions' executive committees had unanimously approved a merger at "all practical speed."

They said the merger would become effective upon approval by special conventions, to be called probably between mid-April and late June of next year.

According to Wynn and Poole, the merger will "increase our capacity to organize the unorganized within our jurisdiction . . . improve our bargaining position on behalf of the members . . . provide for internal resolution of jurisdictional disputes . . . increase the political effectiveness of the unions . . . give us a greater voice within the labor movement . . . enable us to make more efficient and effective use of the combined resources of the two international unions."

No name has yet been chosen for the merged union, whose components' histories date to the 19th century. The president of the retail clerks, the larger of the two, will take over as president. Other leadership positions will be spread between the two.

Yesterday's announcement climaxed several years of merger talks involving several food industry unions and a gradual trend within the labor movement as a whole toward consolidation of fragmented unions, especially smaller ones.

Other major mergers in recent years - all smaller than this one - saw clothing and textile workers combine into the Amalgamated Clothing and Textile Workers Union and the Steelworkers take over the catchall District 50 of the United Mine Workers. There have also been mergers in the rail and printing industries and tasks are underway in a number of others, including the maritime industry.

Unions have also branched out from their principal jurisdictions into other fields. The Teamsters have spread into everything, including organizing policemen.

In their joint announcement, Wynn and Poole noted that the new clerks-meatcutters unions will represent not only the "food industry from field to store" but also employes in "retailing, manufacturing, health care, commerce and financial institutions and other fields."