NEW YORK (AP) _ A newspaper reported federal prosecutors have found evidence indicating that Mayor David Dinkins backdated a letter concerning his sale of cable TV stock to his son, as his mayoral opponent alleged last year.

Mayoral spokesman Albert Scardino denounced the story in New York Newsday Sunday as ''shameful and dishonest journalism'' based on ''unfounded allegations'' made during the campaign.

The letter became a campaign issue when Dinkins' GOP opponent, Rudolph Giuliani, questioned whether Dinkins had paid appropriate federal gift taxes on the transfer of stock in a cable TV company to his son.

In 1986, Dinkins said he transferred 588 shares of Inner City Broadcasting Corp stock stock to his son, David Dinkins Jr., at no cost.

After Giuliani's accusation, Dinkins said he had sold the stock to his son and thus wasn't subject to gift tax. He released copies of the letter from his son - dated Oct. 30, 1985 - as proof of the sale.

Newsday, quoting an anonymous source close to a grand jury that investigated the case, said evidence indicates the letter may have been written in 1988 or 1989, though investigators were unable to prove it conclusively.

In the unnotarized ''Dear Dad'' letter, written by hand on lined paper, Dinkins' son promised to pay his father $58,000 plus 8 percent annual interest by January 1991 for the stock. No payments were made as of last year.

Tests suggested the letter bore the imprint of a telephone number written on another sheet of paper, the source said. The number was of a media company the younger Dinkins was unlikely to have had dealings with in 1985, when he was a CBS sports producer, the paper said.

He now is an independent television producer, and would have had reason to dial the telephone number in 1988 or 1989, the source said.

The mayor's son, who lives in Las Vegas, declined Newsday's request for an interview.

The federal investigation into the stock transfer ended in July without criminal charges. But a city probe continues.

U.S. Attorney Andrew Maloney, in a confidential report summarizing his findings for a special city investigator this July, said he found ''substantial evidence'' the letter was not written when dated, Newsday said.

Maloney, interviewed by the paper Friday, declined to comment on a grand jury matter.

Scardino, the mayor's spokesman, dismissed the story as a rerun of unfounded campaign allegations.

''It was garbage when it was put out then,'' Scardino said. ''It was thrown in trash can at Attorney General's office but now five months later (Newsday is) republishing the same piece of garbage.''

Dinkins' attorney, Charles Stillman, called for a probe into how the paper obtained information on a federal investigation.