The nation's labor laws have "collapsed" and workers are often denied basic legal rights to unionize, largely because of corporate resistance and illegal tactics by employers, a House subcommittee concluded yesterday.

Without reforms, labor relations will deteriorate to the "disruption, upheaval and productivity losses" that characterized the 1920s and '30s and prompted passage of the Wagner Act in 1935, the House Education and Labor subcommittee on labor-management relations said in a strongly worded 58-page report entitled "The Failure of Labor Law -- A Betrayal of American Workers."

"The failure of the law has reached crisis proportions, and those who rely on the Wagner Act for protection are badly deceived," said subcommittee Chairman Rep. William Clay (D-Mo.), who said he will propose reforms in the next session of Congress.

Republicans on the subcommittee issued a minority report calling the Democrats' conclusions overstated and unsubstantiated. They said the recent contracts negotiated by the United Auto Workers and the United Mine Workers, coupled with a decrease in strikes this year, were evidence that labor law is functioning adequately.

The minority report acknowledges problems with the law but accuses the Democrats of using "contrived statistics" and of blaming the Reagan administration for the failure of unions to expand their base.

The majority report, based on a series of hearings in June, follows widespread criticism of the National Labor Relations Board by the AFL-CIO and various unions. The subcommittee report concludes that:

*More than 5 percent of workers who sign union authorization cards are illegally fired for signing. A 1983 Harvard Law Review study said the actual percentage may be far higher because most such illegal firings are never reported to the NLRB.

*The NLRB, despite a shrinking caseload, has an unprecedented backlog; charges of "unfair labor practices" now can take an average of nearly three years before a federal court enforces a finding of illegal activity. Such delays often are caused by intentional and "sophisticated" stalling tactics by employers, the report said.

*When employes vote to unionize, they are successful in bargaining a new contract only about 50 percent of the time, mostly because employers often refuse to negotiate and labor laws provide no enforceable constraints against such resistance.

*Employer resistance to unions is growing rapidly, evident in the "staggering" growth of the "antiunion consultant industry." The report cites AFL-CIO estimates that management is using consultants in two-thirds of all union drives and is spending more than $500 million a year on labor consultants whose aim is to thwart unions.

The report details what it calls Reagan's "politicization of the NLRB" through pro-management appointments, which it says has caused "the collapse of confidence in the objectivity" of the board. "Virtually all the unusually high number of policy reversals of this board have favored management," and management also is winning a dramatically higher percentage of cases, the report said.

Under current Chairman Richard L. Dotson, a former management attorney, the NLRB is two-thirds more likely to dismiss charges against employers as previous boards in the last two administrations, Democratic or Republican, the report said.

The last major labor law reform effort in 1977 passed the House but stalled in a Senate filibuster. Prospects for reform remain dim while Republicans control the Senate, say AFL-CIO officials and House Democrats.