General Motors Corp. and Toyota Motor Corp. tentatively have agreed to give the United Auto Workers Union rights to represent employes of a new company the two auto giants want to set up at a former GM plant in Fremont, Calif.

The agreement represents a breakthrough for the UAW, which so far has failed to organize workers at U.S. plants operated by Honda and Nissan, currently the only Japanese manufacturers building vehicles in this country. At Fremont, the UAW is likely to accept work rules that it doesn't allow elsewhere.

The companies, in an announcement yesterday, also said their joint-venture managers will give preferential hiring treatment to UAW members who lost their jobs when GM closed the plant in March 1982. But the former employes will be hired on the basis of skill, instead of seniority, according to the agreement.

The labor pact, couched in a "letter of intent" signed by joint-venture and UAW representatives, depends on approval of the joint-venture plan by the Federal Trade Commission.

"The letter of intent sets forth understandings between the parties to provide for hiring the very best work force possible, and to establish a collective bargaining relationship whereby the proposed joint venture will agree to recognize the UAW as formal bargaining representatives of future workers" of the new company, William J. Usery Jr., said yesterday. Usery, a former U.S. secretary of Labor, is acting as liaison for GM and Toyota.

Usery denied speculation that GM and a reluctant Toyota entered the agreement with the UAW in an attempt to gain a favorable ruling from the FTC, which has been reviewing the joint-venture proposal for possible antitrust violations.

"We are not here to pressure the FTC," Usery said in a news conference here. "We are here to say we think we have done something that is good.

"We think what we're doing is not only good for America; it's good for our relationship with Japan. It brings jobs to the United States and it's going to bring a car here that General Motors needs at the present time," Usery added.

GM and Toyota claim that their joint venture, which would build 200,000 small cars a year, would create 2,500 jobs at the Fremont plant, and another 9,000 jobs in supplier companies. But the proposal has come under fire from Ford Motor Co., Chrysler Corp., American Motors Corp. and others who say the new jobs would not represent a net increase in employment in the domestic auto industry.

The new front-wheel-drive, GM-Toyota subcompact would compete directly with GM's current small-car lineup, the Chevette and Pontiac 1000, opponents of the venture point out. The aging, rear-wheel drive Chevettes and 1000s could be run out of the marketplace, they reason, displacing hundreds of workers in Wilmington, Del., where the Chevettes and 1000s are made.

Ford and Chrysler have indicated that they, too, would seek joint-venture agreements to stay competitive with GM. But GM officials say the complaints are hypocritical, pointing out that Ford and Chrysler already have tried and failed to enter joint-production arrangements with Japanese companies.

At its peak of operation in 1978-1979, the GM Fremont plant employed nearly 7,000. Many of those workers have "gone back East, found other auto jobs, and some have gotten out of the industry altogether and don't want to ever touch the auto industry, again," said John P. Scrempos, former first vice president of UAW Local 1364, which had represented GM's Fremont employes.

The UAW's international officers revoked the local's charter last month, because the local couldn't pay its bills because of its drastically reduced membership, Scrempos said.