President Reagan has told Libyan leader Muammar Qaddafi that he faces "the most serious consequences" if the reported assassination teams strike against any U.S. leader, administration officials said yesterday.

Reagan reportedly sent a message through a third nation to Qaddafi giving some of the evidence that the United States says it has gathered about Libyan "hit teams" that reportedly have entered the United States. Officials would not say what consequences Reagan contemplates should an assassination attempt be made here.

White House officials continued to deliberate their response to Qaddafi yesterday, and are committed to notifying congressional leaders of their intentions by tonight at the latest. Officials have stressed that they are considering economic and political, not military, measures against Libya.

Reagan chaired National Security Council meetings Monday and Tuesday that dealt with the Libyan question, but there was no NSC meeting yesterday and the president took the afternoon off after a morning schedule that concluded with a luncheon with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) yesterday joined several legislators of both parties who have said the threat from Libya is real and serious. He added: "I'm also convinced the president is approaching the matter in a prudent and appropriate way."

Secretary of State Alexander M. Haig Jr. told reporters in Brussels that he is "appalled" by leaks from within the administration on retaliatory steps against Libya being contemplated. An official who asked not to be quoted by name said Haig was concerned about the confidentiality of the national security process.

Haig's spokesman, Dean Fischer, would not identify the particular items that appalled Haig.

It was unclear to what extent Haig was discussing Libya in his first day of meetings with European foreign ministers in Brussels. Informed sources said it probably will be discussed on a highly confidential basis.

CBS News quoted sources as saying five "hit squad" members have been identified and were being interviewed in Mexico. Three appear to be Middle Easterners and one is blond, possibly an East German, CBS said.

CBS also said it was told that the informant who reportedly alerted U.S. intelligence officials to the alleged plot was being interviewed by psychiatrists outside the United States and had passed several polygraph examinations.

In Mexico City, Mexican and U.S. diplomatic officials said they had no knowledge of a "hit team" in Mexico.

In another development, Conoco chairman Ralph Bailey said he expects the administration to make recommendations to the U.S. oil companies that do business in Libya "before the week is out."