The Mormon church has admitted that founder Joseph Smith married about 40 women including a 14-year-old and others who were already the wives of his followers, having maintained for nearly 200 years that he was monogamous.

The Church of Jesus Christ of Latter-Day Saints has tried to gloss over aspects of its history, including the polygamy practiced by Smith and Brigham Young, who helped found Salt Lake City, Utah, the headquarters of the Mormon church.

"Joseph married many additional wives and authorized other Latter-day Saints to practice plural marriage," a church essay entitled "Plural Marriage in Kirtland and Nauvoo" said, noting that "careful estimates put the number between 30 and 40".

The church has been widely criticized for its treatment of women, black people, who it barred from the higher priesthood until 1978, and gays, who were banned from its temples if they were sexually active.

Smith's wives were mostly between the ages of 20 and 40, but Helen Mar Kimball, the daughter of close friends, was "sealed" to him several months before she turned 15.

"Plural marriage was difficult for all involved. For Joseph Smith's wife Emma, it was an excruciating ordeal," the essay, part of a collection issued over the past year, said.

The church, founded in 1830, banned polygamy in 1890 when the U.S. government threatened to deny Utah statehood.