PHILADELPHIA (Reuters) - A Pennsylvania court overturned the conviction of the first senior U.S. Roman Catholic Church official to be found guilty of covering up child sex abuse by a priest and ordered he be freed on Thursday.

Monsignor William Lynn was convicted in June 2012 of endangering the welfare of a child by reassigning a priest with a history of sexual abuse to a Philadelphia parish that was unaware of his past.

That priest, Edward Avery, later pleaded guilty to sexually assaulting a 10-year-old altar boy in the Philadelphia parish. Lynn, who was not accused of personally molesting children, was sentenced to a three-to-six-year prison term.

On Thursday, a unanimous Superior Court of Pennsylvania appeals panel reversed Lynn's conviction and ordered him discharged from prison, saying the case was "not supported by sufficient evidence."

Lynn's attorney, Thomas Bergstrom, said the ruling demonstrated Lynn should never have been prosecuted, and added he expects Lynn to be released within days.

Philadelphia District Attorney Seth Williams said in a statement he would most likely appeal.

Lynn served as secretary for clergy for the Philadelphia Archdiocese from 1992 to 2004 and, in that role, had access to the archdiocese's secret archives - a repository of information on infractions by its priests - according to the court papers.

While not empowered to reassign or remove priests, Lynn was charged with passing information to higher church authorities.

"SPECIFIC KNOWLEDGE"

In 1994, Lynn began a review of sexual abuse by priests, including allegations Avery had groped an altar server repeatedly. Lynn placed Avery's name on a list of priests who were "guilty of sexual conduct with minors," according to the court's decision.

Lynn eventually recommended Avery be placed as an associate pastor and reside in a rectory at a Philadelphia parish. In 1999, Avery twice molested an altar boy in that parish, according to the court papers.

The three-judge appeals panel found Lynn had "prioritized the Archdiocese's reputation over the safety of potential victims."

But it concluded: "There was no evidence that (Lynn) had any specific knowledge that Avery was planning or preparing to molest children" after he was transferred.

Lynn's lawyers argued on appeal that the law he was prosecuted under was not in place at the time of the crimes.

The child endangerment statute in effect when Lynn was secretary applied to "a parent, guardian or other person supervising the welfare of a child under 18 years of age."

The law was amended in 2007 to include those who oversee the people supervising the child, such as Lynn.

In a statement posted on its website, the archdiocese said it had taken steps to assure the safety of children in its care.

"The decision by the Superior Court to overturn this conviction does not and will not alter the Church's commitment to assist and support the survivors of sexual abuse on their journey toward healing or our dedicated efforts to ensure that all young people in our care are safe," it said.

But the advocacy group, the Survivors Network of those Abused by Priests (SNAP), offered a less optimistic perspective.

"Once again, another high-ranking Catholic official who repeatedly endangered kids and enabled predators is escaping punishment," SNAP said in a statement. "If kids are to be safer, we need to hold employers more responsible, not less responsible, for putting innocent children in harm's way."

(Reporting by Dave Warner; Writing by Edith Honan; Editing by Leslie Gevirtz)