A letter by the American ambassador passionately defending President Carter's human-rights policy, published on July 30 Poland's most prestigious newspaper, was barred from the Voice of America (VOA) Polish-language broadcast by intervention of the ambassador himself.

In an Aug. 3 cable to Washington from Warsaw, Ambassador Richard Davies argued that since the newspaper Polityka had "courageously" published the letter, broadcasting the text by VOA would serve no purpose. The VOA's unseasoned director immediately capitulated to the detente-conscious State Department, which instinctively prefers good bilateral relations to the Carter human-rights crusade.

No decision for many years has so infuriated the professional news staff of VOA. If violated strong guidelines giving total news autonomy to VOA, rather than the parent U.S. Information Agency (USIA) - guidelines issued by USIA Director John E. Reinhardt May 4.

The result is an erosion of both VOA's credibility and the President's world crusade for human rights. The Davies letter, responding to Polityka's earlier criticism of Carter's human rights policy, was an emotional but highly effective defense of that policy. The newspaper, obviously waiting for clearance from Communist boss Edward Gierek, held the letter three weeks before publishing it.

The news value of an American diplomat's letter published in a Soviet satellite state is beyond question. Radio Free Europe and Radio Liberty, subsidized by but independent of the U.S. government, both carried much of the letter. The British Reuters News Service published the entire text.

The reason for this extensive coverage is obvious: Publication of the letter in a Communist newspaper was a major political event, signifying either deliberate Polish defiance of Moscow or an important prelude to this fall's Belgrade conference, which will draw a balance sheet on human rights. But the mere fact or publication was on its face a triumph of the President's policy, which deserves maximum exploitation.

Instead, the Voice of America ran merely a 69-word commentary in its Polish-language broadcasts on Aug. 1, containing no verbatim quotations. The appeal from Davies, perhap understandable under a narrow interpretation of his own duties as ambassador, carried the field; the pledge in Reinhardt's guidelines that "VOA will be solely responsible for the content of news broadcasts" was ignored.

USIA officials insist the decision not to run the text of the letter was made by VOA itself. Technically, in the absence of a specific directive issued to VOA, the decision-maker was VOA's new and inexperienced director, Manhattan radio station owner Peter Straus. After Davies' intervention, Straus suggested that broadcasting the text of Davies' letter could wait until VOA received a true copy in Polish.

But VOA editors believe that on the strength of the cable from Davies' embassy, there was inescapable high-level bureaucratic reluctance at USIA to broadcast the next of the letter. When the Polish-language copy finally arrived here last week, neither Straus nor anybody else made a move to change the decision.

Davies a diplomat widely experienced in both the Soviet Union and Eastern Europe, used his influence on an earlier occasion to muzzle the Voice of America's Polish-language broadcast. That was in June 1976, when he fought the VOA's carrying word to Poland about food riots in Polish cities.

But that was during the administration era when VOA was run by a tough-minded station owner from Mobile, Ala., named Kenneth R. Giddens and his deputy, a career USIA officer named Serban Vallimarescu. Davies' advice was not followed, a decision that angered Henry Kissinger's State Department and that may have explained Vallimarescu's demotion to a lesser job soon thereafter. (Five years earlier, however, Giddens had demoralized VOA by killing a broadcast to Greece for diplomatic reasons.)

The permitted intrusion of Davies in the latest effort to silence the voice will provide new congressional support to give VOA complete autonomy. Sen. Charles Percy's amendment to do just that was adopted by a Senate foreign relations subcommittee early this year but died in the full committee. Percy's renewed effort next year will be helped by the case of the missing letter.