The Reagan administration says it will move ahead with major changes in work rules that would base pay increases, promotions and layoffs for white-collar federal workers on performance rather than seniority--unless Congress blocks the changes or comes up with its own plan by Aug. 1.

Officials of the Office of Personnel Management are upset with a Washington Post story last week that, they say, implied that the White House had shelved the controversial performance-based incentive system that would directly affect 300,000 civil servants here.

The story said that the administration had agreed to delay implementing the plan to abolish automatic in-step increases. It said OPM Director Donald Devine had agreed to postpone indefinitely a major change in personnel rules that would devalue seniority and give job performance more weight in federal layoffs. Devine was described as saying the administration had not retreated on its proposals and would implement them if a compromise failed.

Officials said they have decided instead to give Congress time to consider the proposals.

Under the plan, which has drawn heat from unions and members of Congress, seniority would be devalued. Employes' chances of getting within-grade raises and certain promotions and of avoiding layoffs would be determined by their last job performance ratings rather than time on the job.

OPM said The Post's story concerning a meeting of the Senate Civil Service subcommittee implied that the performance plan was being put on indefinite hold. During the hearing, Chairman Ted Stevens (R-Alaska) and three Virginia Republicans, Sens. Paul Trible and John Warner and Rep. Frank Wolf, urged the OPM to modify an existing performance-based merit pay plan that already covers Grade 13 through 15 supervisors and managers.

The angry GOP legislators represent big blocks of feds who are upset with the supervisory merit pay system and who are fearful that the new emphasis on performance could be used by political bosses to fire longtime employes. They have told OPM not to make any rules change on its own.

On the House side, Democrats are working to block the proposed job-rule changes via the appropriation process.

Mindful of congressional unrest, OPM Director Devine agreed last week to give Congress time to review OPMs proposals, or come up with its own. But he said a newspaper account of the Senate hearing made it appear that OPM had indefinitely postponed the changes.

To show that it is still serious, OPM has sent a special bulletin to top officials throughout the government. The bulletin says the story was "misleading and inaccurate" in regards to the length of the postponment. The bulletin said in part:

"Sen. Stevens . . . wishes to attempt to accomplish through legislation the ends contemplated in OPM's regulatory reform proposals, establishing the principle of pay-for-performance for workers not now governed by the merit pay system and giving greater weight to performance in reduction-in-force rules.

"The administration has agreed to delay temporarily our planned implementation of the regulations . . . and Sen. Stevens has set a goal of Aug. 1 for passage of his legislative proposal. If congressional action is not taken by that date, OPM will be able to proceed with implementing its regulatory approach, and will do so."

The special bulletin said OPM would review agency comments on the proposals and prepare "final regulations for publication." It ended by saying that the regulations "have not been withdrawn, nor will they be. We are merely delaying implementation to allow time for a legislative alternative to advance through the Congress."

In another development, the General Accounting Office yesterday said OPM would be acting illegally if it made the work-rules changes without the approval of Congress.