A sect member with nine wives has been convicted of sexually assaulting an underage bride in the first case stemming from the controversial raid on a polygamist compound in Texas.

Raymond Jessop, 38, faces up to 20 years in prison for the sexual assault on the girl, whom he made pregnant when she was just 15.

Jessop is a leading member of the Fundamentalist Church of Jesus Christ of Latter Day Saints, a sect that split from the Mormon church after it abandoned polygamy. His nine wives allegedly include three daughters and two sisters of the self-styled prophet of a breakaway sect, Warren Jeffs, who is now in jail.

The prosecution relied on documents seized in a raid on the sect’s 1,700-acre Yearning for Zion ranch in April 2008, when authorities took 439 children away from their families.

Although the families were later reunited, prosecutors are pursuing a dozen male sect members on charges ranging from failure to report child abuse to sexual assault.

Jessop, who also faces a second trial for bigamy, refused to take his underage wife to hospital when she went into labour in August 2005 because he feared officials would realise she was younger than the Texas age of consent of 17.

Because the victim did not testify, prosecutors used a photo album and documents taken from the ranch to show that she was one of Jessop’s wives. DNA tests showed a 99.9 per cent chance that Jessop was the father of her child.

Eric Nichols, the prosecutor, rejected the theory that she had consented to marry Jessop. “Any act of sexual assault is a horrendous crime, but an act of sexual assault on a child is of such an extreme nature we don’t even consider whether the victim was able — much less did — consent,” he said.

The jury in the small town of Eldorado, Texas, took just over two hours on Thursday to convict.

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