Senate Republicans postponed a confirmation vote yesterday on the nomination of New Jersey construction executive Raymond J. Donovan as secretary of labor, amid reports that a secret "FBI-protected witness" was prepared to testify that he had personal knowledge of cash payoffs for labor peace by Donovan's old firm.

Donovan was asked through an aide last night to comment on the development. But, after speaking with Donovan, the aide said, "I guess he's not speaking to the press."

President-elect Ronald Reagan said he knew "what the evidence is. He [Donovan] has told me this is absolutely not true, and I have every confidence in him."

Nonetheless, Senate Labor and Human Resources Committee Chairman Orrin G. Hatch (R-Utah) put off the scheduled confirmation vote and met with FBI Director William H. Webster to pursue the new allegation. There was no indication of how long the investigation might continue, but Hatch said, "We're not kidding ourselves. There are some problems. We want to do a more than reasonable job of checking them out."

Hatch declined speculation on Donovan's confirmation chances. However, another Republican committee member said, "Personally, I don't believe he's in trouble. But, obviously, some things have to be cleared up. Only time will tell."

The maneuvering on Donovan came as Senate committees voted to approve three other Reagan nominees: Alexander M. Haig Jr. for secretary of state; Jeane J. Kirkpatrick as U.N. ambassador and Terrel Bell as secretary of education. A vote on William French Smith for attorney general was put off until today.

The fresh information on Donovan reportedly came to Hatch's attention as a result of a lead furnished by Sen. Edward M. Kennedy (D-Mass.), the committee's ranking minority member. But Kennedy staff aides declined to furnish any specifics on the charges or even to confirm his involvement.

Other sources, however, said the allegation, which may or may not have involved Donovan directly, forced a re-examination of his previous testimony. The nominee denied that he was aware of any extortion attempts by politicians or Teamsters union officials during his service as executive vice president of Schiavonne Construction Co. of Secaucus, N.J., a major highway and bridge builder.

During that earlier testimony, Donovan said his company had paid $13,000 in 1967 to Irving Kantor, a Neward businessman identified in a court deposition as a middleman who "laundered" payoffs from contractors to organized crime figures. Donovan said he thought the money was a payment for dumping rights on a tract of land Kantor owned.

He also testified that he had not learned until a few days agot that his company had put a "ghost worker" on the payroll in 1977 in order to guarantee labor peace with a Teamsters local in New York. He said he thought the job was required by the union contract, but conceded that it was a "no-show" position.

But when questioned by committee members, Donovan said, "We have never been extorted. We have never paid a payoff."

It is this last statement, according to some committee sources, that is challenged by the new witness -- whose charges, they emphasized, are without corroboration at this point.

Sources at Reagan's transition headquarters last night gave no indication that they thought the nomination was in serious trouble. Late in the day, as rumors swirled around Capitol Hill, Larry Speakes, a transition spokesman, reaffirmed that "the president stands behind the nomination."

Other officials said a second internal check on Donovan's credentials had been made after his appointment in December, when the FBI field report turned up some allegations involving his firm. The recheck, these sources said, found "no evidence of culpability" on Donovan's part.

But Hatch, who took over the Labor Committee chairmanship with the shift of Senate control to the Republicans, told reporters that he was determined "not to do a sloppy job" in the first major appointment he has handled. Robert Hunter, Hatch's chief of staff, said, "We hope to have it cleared up in a couple days, but we're not going to be rushed or pressed by time constraints."