Major work stoppages were rarer last year than at any time since World War II, according to a Bureau of Labor Statistics report that underscores the decline in union bargaining power and major changes in labor-management relations in the 1980s.

Only 62 major strikes, involving a total of 376,000 workers, began during 1984, compared with the previous low of 81 that idled 909,000 employes in 1983, according to the report on strikes involving 1,000 or more workers.

The number of large strikes, workers involved and percentage of workdays lost because of work stoppages last year were at all-time lows since the BLS began gathering such data in 1947. The figures, however, do not present a comprehensive picture, because in 1981 the BLS stopped keeping track of smaller strikes, which have numbered as many as 5,000 annually.

Labor experts attribute the decline in major strikes to decreased union membership, high unemployment and increased automation that makes strikers more easily replaceable. They also cite more cooperative and "sophisticated" relationships between larger unions and employers and increased global competition in major industries that makes labor and management fearful of strikes.

"It takes a damn fool to strike in a slack labor market, with high unemployment, against a company that might have full inventories," said John Zalusky, an AFL-CIO collective-bargaining specialist.

The statistics reflect a growing realization within organized labor that traditional strikes usually damage both sides, especially in highly competitive industries in which many of the major union contracts are bargained. The United Mine Workers of America, for instance, reached its first strike-free settlement in 20 years last August.

Employers in the 1980s are in a more powerful position because unions, which represent about 19 percent of the work force compared with 35 percent two decades ago, no longer have broad representation within key industries, said Audrey Freedman, labor economist for the Conference Board, a business research group.

"It is very hard for unions to mount an effective strike against an employer if it doesn't have most of that industry organized," she said. "It is also hard because the employer realizes he can hire a new work force because there are plenty of people available."

Union officials also say that President Reagan's replacement of 11,000 striking air-traffic controllers in 1981 set a hard-line tone for employers and created fear among potential strikers.

The number of major strikes dropped below 100 for the first time in 1982, compared with 200 to 400 annually during preceding decades.

The top figures were in 1952, when 470 major strikes idled 2.7 million workers and cost employers nearly 4 percent of total work hours. The lost work-hour figure declined nearly tenfold by 1984, when it was 0.4 percent.

Last year's largest strike idled 91,300 United Auto Workers members at General Motors Corp. for seven days, accounting for 416,000 lost workdays.

The highest number of lost workdays came in the New York City hospital strike in July by 52,000 members of District 1199 of the Retail, Wholesale and Department Store Union. The BLS said 1.5 million workdays were lost in that 46-day walkout.