An independent counsel secretly assigned to examine new allegations against former labor secretary Raymond J. Donovan has decided there is "insufficient corroborative evidence to support a successful prosecution," it was disclosed yesterday.

The findings by Leon Silverman, who conducted what amounted to his third investigation of Donovan, concluded an inquiry into alleged kickbacks of $15,000 supposedly paid to Donovan in 1977.

In a report released by a special three-judge federal court, Silverman said the testimony of two key witnesses, if credited, "could have formed the basis for an indictment."

But he added that he thought a conviction would have been unlikely because of faded memories, "fundamental inconsistences and contradictions" in the witness accounts and "a lack of sufficient credible and corroborative evidence." He recommended against prosecution and a federal grand jury voted unanimously Oct. 13 against indictment.

Donovan declined to testify, informing Silverman through his attorneys that he would take the Fifth Amendment if subpoenaed.

In a statement, Donovan said the allegations were "yet another baseless complaint against me . . . . This chapter in my life is now finally closed."

Silverman delayed his investigation for two years, at the special court's direction, "to avoid publicity" while Donovan's trial on fraud and grand larceny charges was pending. Donovan was acquitted last May 25. Silverman began his inquiry eight days later.

The allegations were made by Gerald Meyer, president of Columbian Steel Co. of Little Ferry, N.J., and coowner Ronald Singer, who told authorities they arranged in 1976 to supply steel for New York City subway projects underaken by Donovan's firm with the understanding that the bid would be inflated by $250,000, "the bulk of which was to be paid as a kickback to Donovan."

In December 1977, according to the report, Meyer said he and Singer "decided to make a good-faith payment" and that he delivered $10,000 to Donovan.