The head of the Federal Aviation Administration said yesterday his agency will move vigorously against future slowdowns by air traffic controllers such as the slowdowns that snarled traffic at Washington National and other airports last May and June.

The decision, announced by FAA Administration Langhorne Bond at an early morning meeting with reporters, is designed to discourage future slowdowns, which in recent years have sometimes coincided with holidays, such as next week's long Thanksgiving weekend.

Bond released a letter he had received in which the Justice Department said it has decided against prosecuting "the individuals involved [in the May-June slowdown] as well as certain Professional Air Traffic Controllers Organization (PATCO) union officials."

At the same time, Justice said, "sufficient evidence may exist to support criminal prosecution" under federal statutes forbidding strikes by federal employes. The air traffic controllers are FAA employes.

PATCO has already been assessed $100,000 in fines and found in contempt of court under civil proceedings stemming from the slowdown. In a footnote to his July decision assessing those fines, federal District Judge Thomas Platt of Brooklyn noted "it is also the sworn duty of the attorney general to enforce these laws, but for reasons not fathomable to this court, they had yet to initiate any investigative or enforcement proceedings."

The FAA and Justice got the hint. Although Justice declined to prosecute in this case, it has set out a series of procedures the FAA should follow in the case of future slowdowns. Those steps include injunctions, comtempt citations and disciplinary proceedings under civil service laws. If they fail to end a slowdown, "criminal investigations . . . will be undertaken," the letter said. It was signed by Philip B. Heymann, assistant attorney general in the criminal division.

Bond conceded yesterday that it is difficult to prove that a controller is actually slowing things down. "There are a lot of little subtle ways controllers can expedite traffic or slow it down . . . You know there are delays but the casual connection is hard to make."

Nonetheless, he said, he would be less reluctant "to call a slowdown a slowdown" when one came in the future. The FAA has traditionally been reluctant to confirm slowdowns even when it was obvious to everyone flying that they were in progress. "We have been too reluctant to believe other sources," Bond said.

He said he believed that the May-June slowdown "was enormously costly to the union" and that "my opinion is that a slowdown is no longer a productive tool!"

The slowdown in question came after PATCO warned that such actions were possible if airlines failed to permit controllers to make free unconditional "familiarization" flights to Europe and the Far East. Such flights on domestic routes are commonplace.

PATCO took unusually harsh criticism for the slowdown on editorial pages in aviation journals that have tended to be sympathetic to controllers and in Congress.

Bond implied that the slowdown may have cost the controllers their taxpayer-funded, second-career program, which was eliminated by the last Congress. Under that program, which was criticized by the General Accounting Office as extremely wasteful, controllers were retrained for new occupations at taxpayer expense.

Robert Poli, PATCO vice president, declined to comment on the Justice Department letter. He said that the GAO report, not the slowdown, was the reason Congress rejected second-career program.

The Justice Department declined to seek prosecution, according to Heymann's letter "because of a long-standing policy . . . not to preliminarily initiate criminal prosecution under the antistrike statute . . ." Such action had never been taken in the 23 years of the law, Justice said, and a change from that policy "should be preceded by notice . . . to any individuals" who might someday be charged.