Federal efforts to prosecute Airlie Foundation director Murdock Head for his role in an alleged bribery scheme were thrown into doubt yesterday by a judge's order limiting key evidence that may be presented at Head's trial.

U.S. prosecutors asserted that they would be barred from proving six tax-evasion and tax-falsification charges on which Head was indicted unless Judge Albert V. Bryan Jr. agrees to modify his ruling.

In addition, U.S. Attorney Justin W. Williams told Bryan during a brief court session late yesterday afternoon that his order "may impact not only on the tax counts, but the conspiracy [count] -- I may not have a case to try." The conspiracy count is the principal charge that prosecutors have leveled against Head.

Dr. Head is scheduled to go on trial Tuesday on charges that he conspired to bribe Rep. Daniel J. Flood (D-Pa.), former Flood aide Stephen B. Elko, former Rep. Otto E. Passman (D-La.) and a former Internal Revenue Service agent in an attempt to obtain federal grants, contracts and favorable treatment for his foundation and other ventures. Airlie Foundation is based in Warrenton, Va.

In announcing his order in District Court in Alexandria yesterday morning, Bryan sharply criticized the prosecutors for their handling of a legal stipulation that is central to the dispute. A stipulation is a court-approved agreement between prosecutors and defense lawyers that certain statements, documents or other evidence are indisputably accurate.

"The next time you sign a stipulation, read it -- because you never sign a government's case right out of the window," Bryan told the prosecutors.

In his complex ruling, Bryan held that the prosecutors would be blocked from trying to prove that certain documents were false because the prosecutors and Head's lawyers had previously agreed to stipulate that these were accurate.

After Bryan issued his order, the prosecutors said they discovered that the documents barred from attack under this stipulation included tax returns that are central to the six tax-evasion and tax-falsification charges. If they cannot argue at Head's trial that these tax returns are false, the prosecutors said they "could not prove" that Head is guilty of tax offenses.

Williams did not say why Bryan's ruling might jeopardize the prosecutors' attempt to prove the main conspiracy charge against Head. Nevertheless, the tax and conspiracy counts listed in Head's indictment are partly intertwined. It appeared possible yesterday that the prosecution feared it would be barred from presenting evidence about tax transactions that are linked to the conspiracy count.

After listening to Williams' plea for a modification of his ruling, Bryan agreed to schedule another court hearing on the issue for Monday morning. Bryan did not indicate, however, whether he would alter his order.

Williams said during yesterday afternoon's court session that the prosecutors may decide to appeal Bryan's ruling to the Court of Appeals in Richmond, if Bryan does not agree to modify his order. Such a move would almost certainly delay the start of Head's trial.

Neither Head nor his lawyers were present during the 5:25 p.m. court session yesterday. Ond of Head's lawyers, Frank W. Dunham Jr., said later in a telephone interview that they probably could attend a court hearing on Monday. It was unclear what would happen if Head's lawyers were unable to appear in court on Monday. When asked about this prospect, Williams said, "I don't know."

Bryan is the second judge to have criticized the prosecutors during pretrial hearings in the Head case. Judge Oren R. Lewis has repeatedly lambasted the prosecutors, accusing them at one point of making "a deliberate misstatement of fact" about his rulings in the case.