Three Southern California researchers say they have new evidence that challenges the authenticity of the Book of Mormon, one of the sacred writings of the Church of Jesus Christ of Latter-day Saints.

Based on the opinions of three handwriting experts, the researchers have declared that portions of the Book of Mormon were written by a Congregationalist minister and novelist who died more than 10 years before Joseph Smith is said to have received the revelations from God through golden plates.

Though controversy about authenticity of the Book of Mormon has swirled since its publication in 1830, the critics' case until now has rested on circumstantial evidence.

Critics had maintained that similarities of style, subject matter and testimonies of perhaps biased persons linked Smith, founder of the Mormon Church, with Solomon Spaulding, the minister-writer who died in 1916.

Enlarged Photocopies

But this week young researchers, none of whom is now a Mormon, revealed that they believe Spaulding wrote 12 pages of "First Nephi," part of the 522-page Book of Mormon.

Asked for comment, a press spokesman for the Mormon Church in Salt Lake City flatly denied that any of the pages of the Book of Mormon were written by Spaulding.

The researchers, Howard A. Davis and Donald Scales, both of Torrance, Calif., and Wayne L. Cowdery of Orange, Calif., say that two years ago they obtained enlarged photocopies of 12 original manuscript pages that are in the Latter-day Saints archives in Salt Lake City.

These reproductions were compared with specimen of handwriting in "Manuscript Story," a novel about the origin of American Indians generally acknowledged to have been written in longhand by Spaulding around 1812.

The handwriting analysts, all well known in their field, worked independently and did not know the Book of Mormon connection, Cowdrey said in an interview.

'By Same Writer'

The first expert to be consulted was Henry Silver. He told the Los Angeles Times: "It is my definite opinion that all of the questioned handwriting... were written by the same writer known as Solomon Spaulding..."

Silver has analyzed thousands of cases, including the so-called Mormon will of Howard Hughes, which was found last year in the world headquarters of the Mormon Church in Salt Lake City.

Silver, 86, emphatically stated then-as he still did this week-that the writer of the will was none other than Hughes. Other experts have loudly denounced the will as a fake.

The other handwriting analysts who examined the Spaulding materials and the reproduced Mormon pages were Howard C. Doulder and William Kaye. Both live in the Los Angeles area and are frequently called to testify in court cases.

Doulder told the Times, "This is one and the same writer," assuming that the photocopied material he was furnished is a true copy of the original documents in Salt Lake.

'Considered Opinion'

Kaye, in an opinion written Aug. 27, 1976, said it was his "considered opinion and conclusion that all of the writings were executed by Solomon Spaulding..."

Kaye told the Times he had been asked to ascertain the probable point in time when the documents had been written, but he could not do that without seeing the originals.

Doulder said he has been a questioned documents examiner for the Milwaukee Police Department and the U.S. Treasury Department, as well as serving as chairman of several national document and identification organizations. Kaye said he has done similar work for business firms and criminal justice agencies here and in Canada.

The researchers first old the story of their investigation to Edward E. Plowman, news editor of Christianity Today magazine. The Protestant journal will carry a lengthy account of the matter in its July 8 issue.

Critical Controversy

The controversy is a critical one for the Mormons, a fast-growing church of 3.8 million members worldwide, because they believe the Book of Mormon is a divinely inspired and correctly translated work of God.

"If the book is every proved to be something other than what Joseph Smith claimed, the church's foundation itself will be in question." Plowman observes in his Christianity Today article.

Don LeFevre, the LDS spokesman, said that no handwriting analysis of the questioned documents will be made by the church.

"There's no doubt in our mind that the Book of Mormon is anything other than what we've always said it is," he said. He invited a Times reporter to examine the 20 laminated pates in the archives from the so-called Kimball acquisitions.

Unidentified Scribe The 12 pages reproduced from that collection and examined by the handwriting analysts were dictated by Smith to "an unidentified scribe," according to Mormon historians. Two parts in the collection (the remaining eight pages) have been ascribed to the hand of two close associated of Smith.

Plowman said the Kimball acquisition pages appear to be in sequence, and the paper seems to be uniform in size, stock, and age, though the handwriting seems to differ from section to section.

According to Smith's official church writings, the Book of Mormon is a miraculous translation of "reformed Egyptian hieroglyphics" on golden plates he dug out of a hillside in 1827 near Palmyra, N.Y. Besides Smith, a few others saw the plates before the Angel Moroni took them away after making sure the translation was correct, Mormons teach.

Unanswered Questions

A number of questions about the pages linked to Spaulding by the handwriting experts remain unanswered. And the research project itself is not altogether a disinterested study.

Why a scribe would insert pages written by Spaulding into the Book of Mormon manuscript instead of rewriting them is open to conjecture.

"The sections in the archives appear to have been written at the same time with the same ink on the same stock of paper," said LeFevre. "Why would Smith take the original manuscript and try to match the ink and paper-It would have been easier to copy off Spaulding's writing in his own hand if he had wanted to plagiarize."

Researcher Cowdrey speculates that Smith had Spaulding's notebook in which the manuscript was written and simply had the other scribes write new material on unused pages of the notebook as Smith dictated. The, Cowdrey reasons, Smith tore out some of the pages written by Spaulding and all of the pages newly written by the scribe and put them in their present sequence.

Explain Uniformity

This, says Cowdrey, would explain the uniformity in the paper stock and its age.

Cowdrey and Silver plan to examine the Kimball collection at the Salt Lake archives on Tuesday, he said. Silver said he can tell whether the ink throughout is of the same age.

Doulder said he had not yet been contacted about going, but that he would only go separately from the other two analysts in order to make an independent examination.

Kaye said that he had been approached about examining the documents, but no date had yet been set.

The researchers have written about their investigations in a book to be published this summer by Vision House, an evangelical publishing firm in Anaheim, Calif.

Paid the Fees

It was also learned that Walter Martin, a staunch defender of conservative Christianity against cults and followers of the occult, paid the fees of two of the three handwriting analysts.

In his book, "Kingdom of the cults," published long before the handwriting study was made. Martin says that Spaulding is the author of parts of the Book of Mormon.

A multi-million dollar civil suit by Martin against the Mormon Church is pending in Orange County (Calif.) Superior Court. Martin, who heads the Christian Research Institute in Anaheim, says that while he was delivering a lecture he was interrupted by a Mormon official who accused him of deceiving people and distorting LDS doctrine.