Warren Jeffs, the leader of the Fundamentalist Mormon church that practises polygamy, is likely to spend the rest of his life behind bars after he was given the maximum sentence for taking girls as young as 12 to be his brides.

Jeffs, 55, who represented himself during most of his trial though he refused to attend the final stages, claimed that his prosecution was a violation of his religious rights and warned it would lead to "sickness and death" being brought on the locality.

But a Texas jury took just 40 minutes to hand him the toughest sentence open to them – life in prison for aggravated sexual assault and 20 years in prison for sexual assault of a child. The sentences followed Jeffs's conviction last week on two counts relating to polygamist marriages with girls aged 12 and 15.

The sentencing of Jeffs brings to an end the most high-profile prosecution yet of a member of the Fundamentalist Church of Jesus Christ of Latter Day Saints. The church, which has been based in Eldorado in west Texas since 2004, claims to be the genuine Mormon church adhering to the traditional practice of polygamy, or plural marriage as it calls it.

The official Mormon church broke with the practice of polygamy in 1890 under pressure from the federal US government which made renouncing the practice a condition of granting statehood to Utah.

The polygamist Mormons, under Jeffs's leadership, refused to renounce plural marriages and now have about 10,000 followers. The jury heard that Jeffs personally had about 78 wives, including 12 who he married at 16 and 12 who he married at 15 or younger.

Jurors were played audio tapes which the prosecution alleged recorded Jeffs giving sexual instructions to several underage wives. One tape was said to have captured the sounds of Jeffs having sex inside the temple at the church's Yearning for Zion Ranch in Eldorado.

The court was also presented with DNA evidence that showed Jeffs fathered a child with a 15-year-old girl.

The jury heard that Jeffs was an overweening presence within his church.

When fathers resisted handing over their daughters, Jeffs would expel them – he ejected 60 church members and broke up 300 families.

As the carrot to the stick, he encouraged fathers to give away their teenage daughters by offering them their own young brides.

Jeffs dismissed several teams of defence lawyers and in the end represented himself in his own unorthodox style. Last Friday he gave a speech in which he accused the court of mocking his religion and read from a piece of paper which he said carried Jesus's own words: "I will wrest your power. I shall judge you. I shall let all peoples know your unjust ways. I will send a scourge upon the counties of prosecutorial zeal to be humbled by sickness and death," he read.

In the course of his defence, Jeffs said polygamy was a "pure, natural way of life" and one of "God's laws". He pointed out that it was a central element of Mormonism in the early days of the church, which was founded by Joseph Smith in New York state in the 1820s.

Smith himself is thought to have had at least 30 wives.