The U.S. comptroller general said yesterday that the Reagan administration's proposal to abolish automatic "in-step" pay increases for an estimated 1.2 million federal employes violates a 1945 law that abolished a practice similar to the one the administration is proposing.

The Office of Personnel Management's proposed rule would establish a two-tiered system: employes in the top three of the 10 "steps"--the rungs of the ladder between one civil service grade and the next--must have performance ratings of "exceeds fully successful" or better before they can advance to the next step. Employes in the first seven steps of any grade would have to achieve "fully successful" ratings.

Now, the same rating system applies to all federal employes below the GS-13 level who are eligible for "step" promotions--and for the 3 percent raise that goes with them. Workers in the higher steps of any grade are usually those with the longest tenure in the federal work force.

By setting a higher standard for some employes, Deputy Comptroller General Milton J. Socolar said the OPM failed to heed "the governing statute that requires that the same performance standard or rating apply to all of the 10 steps of the grades within the general schedule."

In a letter requested by Rep. William D. Ford (D-Mich.), chairman of the House Post Office and Civil Service Committee, Socolar said that in 1945 Congress "specifically rejected" a two-tiered system for judging employe performance.

OPM Director Donald J. Devine yesterday said the letter was "a monumental misreading of the law." In 1962, he said, Congress enacted a law "giving the obligation to the agencies and the Office of Personnel Management and the president to . . . determine an acceptable level of competence. Once Congress delegates an authority to the president, its precedents no longer stand . . . .

"Congress gave the authorization to the president, and he gave it to me," Devine added, referring to a 1973 executive order delegating certain personnel decisions to the head of the federal civil service.

Last week, nearly two months after he made the controversial pay proposal along with a package of others designed to de-emphasize seniority and link pay and job security more closely to job performance, Devine agreed to a congressional request to hold off on a final rule on in-step pay raises until Aug. 1, to give legislators a chance to enact legislation on the same subject.

"If we Congress and the Reagan administration arrive at something that achieves our goals, there's no reason to go ahead with the regulation," Devine said.

But if nothing is worked out by Aug. 1, he said, "we publish the regulations" in final form.

Socolar's letter to Ford fuels the debate over the pay proposals, which had been criticized by Democrats like Ford and Republicans like Rep. Frank R. Wolf (R-Va.). In a news release publicizing the Socolar letter, Ford said it "puts this administration on notice that it cannot unilaterally change the law in pursuit of its reckless goals.

"This administration, and especially . . . Devine, has displayed monumental arrogance in their assault on federal employes. This opinion should serve to warn them that they have gone too far."