The administration yesterday announced its long-promised and controversial plan to ease job bias regulations affecting employers who do business with the federal government and to reverse a regulatory trend of nearly two decades.

The proposal would exempt employers with fewer than 250 workers and a contract worth less than $1 million from the requirement that they prepare any written plan for hiring and promoting women and minorities. Current rules cover any firm with 50 or more workers and a $50,000 contract.

"The changes we are proposing will create incentive for voluntary compliance and put an end to mindless confrontations with employers who have been acting in good faith," said Ellen M. Shong, director of the Labor Department's Office of Contract Compliance Programs, which is to publish the new rules in today's Federal Register.

In a public breach unusual in the administration, one agency with jurisdiction over the new plan has refused to endorse it. The Equal Employment Opportunity Commission, which has approval rights under the law, has registered official objections to at least some parts of it.

EEOC officials refused to comment, but informed speculation is that their reservations are similar to those of civil rights groups, who have objected strenuously to raised thresholds that eliminate many smaller employers from certain requirements. If the differences are not resolved, the White House will make the final decision.

Shong also declined to discuss the disagreement, characterizing it as a "continuing consultation" that may dissolve.

The plan, which replaces a set of changes initiated just before the Carter administration left office, was announced by Labor Secretary Raymond J. Donovan. He said the package "keeps the necessary safeguards for protected groups, while cutting down the paper work burden for employers."

The regulations cover 30 million American employes in about 200,000 firms with federal contracts.

Other provisions in the plan would:

Eliminate various paper work and reporting requirments, including a Carter administration requirement that employers write a summary of their affirmative action programs.

Exempt employers from setting goals and timetables for job groups in which employment of women and minorities is at least 80 percent of their availability in the general work force.

Permit contractors with 250 to 499 employes to prepare an abbreviated affirmative action program.

Eliminate pre-award reviews of employers due to receive federal contracts of at least $1 million.

Reduce from 16 to nine the number of affirmative action steps required of construction contractors and apply the steps, goals and timetables, only to the larger contractors.

According to Donovan, raising the thresholds for the written affirmative action programs to include only larger firms will free nearly 75 percent of employers from this burden while retaining protection for almost 77 percent of workers currently covered.

Employers with contracts worth at least $10,000 still are subject to the executive order requiring federal contractors to take affirmative action to hire and promote women and minorities, whether or not they must prepare a written plan, Donovan said.

While the new proposals have upset advocates of affirmative action, they have drawn criticism from some business groups, which complain that the new conservative administration has stopped short of the relief they had expected.

For example, the administration has called only for more public comment on the issue of back pay as a remedy for discrimination. Some business interests had hoped it would be eliminated.