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UPDATE 6-Ex-Vatican treasurer Pell returns to jail after losing appeal against sex abuse convictions

* Court says Pell victim was compelling, 'a witness of truth'

* Jury verdict upheld as reasonable based on evidence

* Pell returns to prison for at least three years

* Pell maintains innocence, legal team may launch final appeal

* Vatican "acknowledges" decision, says Pell has right to appeal (Adds second Vatican statement)

By Sonali Paul

MELBOURNE, Aug 21 (Reuters) - Former Vatican treasurer Cardinal George Pell will remain in prison for at least another three years after losing an appeal against his conviction for sexually abusing two 13-year-old choir boys, an Australian court ruled on Wednesday.

Pell, the highest ranking Catholic worldwide to be convicted of child sex offences, was jailed for six years in March after being found guilty on five charges of abusing the two boys at St Patrick's Cathedral while he was Archbishop of Melbourne in the late 1990s.

Supreme Court of Victoria Chief Justice Anne Ferguson said two of the three judges hearing Pell's appeal "decided that it was open to the jury to be satisfied beyond reasonable doubt that Cardinal Pell was guilty of the offences charged" and rejected his appeal.

Under the terms of his sentencing, Pell will be eligible for parole in October 2022, when he will be 81.

Pell, wearing a black suit with a priest's collar, sat quietly in the court with no discernible expression as Ferguson read the judgment.

The jury in his trial heard testimony from a victim who described how Pell had exposed himself to the two boys, fondled their genitals and masturbated, and forced one boy to perform a sex act on him. The other victim died in 2014.

"I am grateful for a legal system that everyone can believe in, where everybody is equal before the law and no one is above the law," the surviving choir boy, now in his 30s, said in a statement read out by his lawyer, Vivian Waller.

"The criminal process has been stressful. The journey has taken me to places that, in my darkest moments, I feared I could not return from," he said in the statement.

Pell's spokeswoman, Katrina Lee, said he maintained his innocence and his legal team was examining the judgment to determine whether to lodge a special leave application to the High Court of Australia to hear a final appeal. Pell has 28 days to file the application.

"Cardinal Pell is obviously disappointed with the decision today," Lee said.

The Vatican said it acknowledged the court's decision and remained committed to pursuing clergy who carried out abuse.

"As the proceedings continue to develop, the Holy See recalls that the cardinal has always maintained his innocence ... and that it is his right to appeal to the High Court," the Vatican said in a statement.