An FBI informant who has been described as "extremely reliable" has made new allegations concerning Labor Secretary-designate Raymond J. Donovan that the bureau did not investigate, according to Democratic members of the Senate Labor Committee.

The individual reportedly gave his information to the FBI last Monday, but according to a minority report on Donovan's nomination, the FBI "conducted no investigation of the allegations."

The informant asserted that Donovan's company, the Schiavone Construction Co. of Secaucus, N.J., "had a reputation of having ties with the Genovese organized crime family," the report states. The source mentioned three members of the Genovese group in particular.

In addition, according to a draft of the report, the same informant "further alleged that he heard Raymond Donovan's name mentioned in a conversation as a contractor who obtained state contracts by bid rigging on behalf of the Schiavone company."

Two other men were said to have been present during the alleged conversation.

Apprised of the Democratic report, FBI officials reiterated yesterday that they are satisfied their investigation was complete and they do not intend to reopen it.

"It has to stop somewhere," said FBI executive assistant director Francis M. (Bud) Mullen with a trace of exasperation. "No new information has come to our attention that warrants further investigation."

Donovan has denied any wrongdoing and dismissed his detractors as "hoodlums" mouthing groundless accusations. His nomination is scheduled to come up for debate on the Senate floor Tuesday.

FBI officials insisted at a hearing last Tuesday that they had taken their investigation as far as it could go. They said they were unable to corborate any of the allegations against Donovan despite more than 100 interviews, laborious inspection of company records, and even a review of electronic surveillances of organized crime figures in the New York-New Jersey area in recent years.

Sen. Edward M. Kennedy (D-Mass.), the ranking Democrat on the Senate Labor Committee, has, however, asked the Justice Department to make the FBI informant whose allegations were not followed up available to committee investigators tomorrow.

Sources said the request was addressed to Deputy Attorney General Charles B. Renfrew since the man in question is currently in the Justice Department's "protected witness" program.

The Democratic report, signed by Kennedy and four Labor Committee members who said they were troubled by Donovan's nomination, said the informant "is acknowledged by the FBI to be reliable."

"A former Justice Department official who dealt closely with this source describes him as extremely reliable," the report continued.

No reason was given in the report for the FBI's reported failure to follow up what he had to say, but FBI officials maintained at last Tuesday's hearing that it was not detailed enough.

". . . he had heard that Schiavone Construction had organized crime connections," Mullen told the committee in a brief allusion to the informant in question. "He did not have any information on Mr. Donovan. He was unable to provide any specifics, that is identify individuals or specific organized crime connections."

The Democratic report, however, said the informant "mentioned the following three members of the Genovese family as having ties with the Schiavone company: Joey Adams, Peter LaPaco and Tino Fiumara."

Fiumara was convicted of racketeering and extortion last May along with several waterfront union leaders in connection with his alleged control of the New Jersey docks.

The head of the FBI's special inquiry unit, Anthony Adamski Jr., also testified briefly at Tuesday's hearing about how the informant "believed he had heard Mr. Donovan's name in connection with bid rigging in the construction industry. Again, nonspecific, no details, and it was only his belief."

The Democratic report suggested that the individual had been more positive than that and also cited the names of two individuals said to have been present at the same conversation.

"The FBI people who testified were not aware of either the names of the Genovese family members or of the two people allegedly present at the conversation about bid rigging," a Democratic source said yesterday.

Mullen said he didn't know whether his men simply thought the additional details unimportant or whether committee investigators obtained them later on their own.In either event, he was unimpressed. "If you're going to come up with a few more names from 10 to 20 years ago, you could go on forever," he said. "I'm satisfied with our investigation."

The Democratic report was signed by Kennedy and Sens. Claiborne Pell (D-R.I.), Thomas F. Eagleton (D-Mo.), Donald Riegle Jr. (D-Mich), and Howard Metzenbaum (D-Ohio). All had voted simply "present" when the Labor Committee recommended Donovan's confirmation by an 11-to-0 vote, and the five Democrats still have not stated how they intend to vote when the issue comes up on the Senate floor.

But they said in their report, which will be published tomorrow along with the views of the rest of the committee, that they consider President Reagan's choice "almost unique in the history of presidential nominations to the Cabinet because of the number and gravity of the allegations against Mr. Donovan."

The major disputes involved the Schiavone company's payment of $13,000 in 1967 to a Newark businessman who later made a deathbed confession that he had been serving as a front man for kickbacks to a New Jersey Mafia leader; the company's employment in 1977 of a "ghost" foreman to maintain labor peace with a New York Teamsters local, and allegations by another government-protected witness, Ralph Picardo, that Donovan had periodically handed him $500 checks in the late 1960s for "labor peace" with Teamsters officials in New Jersey.

Donovan denounced Picardo as "murdering slime." He testified that he thought the $13,000 payment, which he had okayed, was for dumping rights on a lot the Newark businessman supposedly owned. And he said he had been unaware of the "ghost employe" incident until last month even though three Schiavone officials, including a close friend of Donovan, were questioned about it before a federal grand jury in 1978.

In their report, the Democratic senators noted that the lot Shciavone Construction used for dumping was not owned by the Newark businessman, but was apparently a municipal low owned by the city of Newark. They also accused Donovan of inaccurate testimony concerning a facet of the "ghost employe" incident.

Donovan told the committee Jan. 12 that he had just checked with an official of the General Contractors Association in the New York area, William Finneran, and that Finneran remembered telling another Shciavone company executive back in 1977 that the company had to hire whomever the Teamsters designated.

According to the Democratic report, however, Finneran told committee investigators that he did not talk to the Schiavone executive and that he simply told Donovan that someone else at the contractors association might have been consulted. In addition, the report said, "Mr. Finneran advised committee staff that he would advise any contractor not to pay for someone who was not coming to work."