Pilots in four passenger planes have received false radio instructions since the air controllers' strike began from persons impersonating on-duty controllers, the Department of Transportation said yesterday. The instructions were corrected by controllers on duty without mishap.

In three other cases, pilots were harassed over frequencies reserved for ground control. Unidentified voices were heard to say "scabby controllers" and "you are going to be killed," according to Transportation Secretary Drew Lewis.

The disclosures came as the controllers' union suffered another defeat in its legal battle against the government. Yesterday afternoon administrative law judge John H. Fenton recommended that the Federal Labor Relations Authority decertify the union for conducting an illegal strike.

His recommendation grew out of charges of unfair labor practices by the Federal Aviation Administration. Robert Poli, president of the striking Professional Air Traffic Controllers Organization, said yesterday the union plans to file exceptions to the authority. If the authority decides to decertify, the union could take it to the U.S. Court of Appeals here.

Meanwhile, as transatlantic traffic returned almost to normal, Portuguese controllers, who handle about 50 transatlantic flights daily to and from the United States, voted early today to boycott U.S. traffic for 48 hours beginning at 8 p.m. (EDT) Sunday, Reuter news agency reported.

Secretary Lewis said there was "no direct evidence that the recent radio incidents are strike-related." But he emphasized that anyone found tampering with air control communications would be prosecuted to the full extent of the law. The incidents are being investigated by the FBI.

Poli strongly denied that the union was involved, saying it had specifically instructed members to refrain from violence or acts of that sort. "It's not us," he told a press conference.

Certain frequencies on VHF radios are reserved for use in air control. Aviation radios can be purchased without restriction and are standard equipment aboard aircraft of all sizes.

On Aug. 4, the day after the strike began, an Air Florida airliner was approaching a runway at Ft. Lauderdale airport when a radio voice instructed its pilot to "please go around," meaning to break off approach and circle, according to DOT. But controllers in the tower countermanded the order and the plane landed safely.

That same day, an FAA spokesman said, an airliner on approach to LaGuardia airport in New York was also told to turn. Again, the instructions were reversed by the tower.

An FAA spokesman said that before the strike such interference was extremely rare. The last case in memory came some months ago when pilots flying in the Tampa area received false messages. Investigators never located the source.

Procedures whereby pilots repeat to the tower any instructions they receive provide some safeguards against such outside interference. In addition, if false messages reach an airplane, they will often reach the plane's real controller as well, who is monitoring the same frequency.

The FAA reported that traffic by domestic airlines using the 22 major airports under "flow control" restrictions remained at about 75 percent of normal. Traffic across the Atlantic was back to levels it had maintained before the Canadian controllers' two-day boycott.

Staff writer Martha Barnette contributed to this article.