Federal prosecutors, responding to a request by Ivan Boesky for a reduction in his three-year prison term, say the former top speculator's sentence was "appropriate."

However, the Manhattan U.S. attorney's office did agree to Boesky's request that the judge in the case defer any decision on his motion until October.Boesky, who was at the heart of the largest insider trading scheme in U.S. history, says that he has given significant new information about other corruption in the securities industry since his sentence.

He claims that in six months this cooperation will become evident as a result of new indictments.

"In the government's view, Boesky's three-year sentence was and remains an appropriate sentence in light of all the competing factors," said Asst. U.S. Attorney John Carroll in his affidavit.

Carroll also said that Boesky "correctly states both that he has continued to provide cooperation to the government and the government's investigations relating to the information he has provided continue."

The prosecutor also requested that the government be given an opportunity to give its views on the sentence reduction before U.S. District Judge Morris Lasker makes his ruling.

In a court filing last week, Boesky told Lasker, who sentenced him, that the "extraordinary extent" of his cooperation with government investigators will almost certainly result in additional criminal and civil charges against others.

Boesky, 51, was sentenced Dec. 18 on one criminal conspiracy charge. He had pleaded guilty in April 1987.

He began serving the term on March 24 in a so-called "country club" prison in Lompoc, Calif., and is eligible for parole at the end of one year.

Boesky's illegal activities were made public on Nov. 14, 1986 when he agreed to pay $100 million to settle civil insider trading charges brought by the Securities and Exchange Commission.

At that time, he also agreed to plead guilty to one criminal charge. Boesky won leniency from the government by cooperating in its continuing probe of insider rings, and he reportedly taped conversations with his business contacts.