By Kevin Murphy

KANSAS CITY, Missouri (Reuters) - The Roman Catholic bishop in Kansas City has asked a court to dismiss charges he failed to report suspected child abuse by a priest on grounds he was not mandated to do so.

Bishop Robert Finn, leader of the 133,000-member Diocese of Kansas City-St. Joseph, is scheduled to go on trial in September in Missouri's Jackson County Circuit Court on charges he failed to inform authorities for months that Father Shawn Ratigan had child pornography on his laptop computer.

Ratigan pleaded not guilty to felony child pornography charges last year. Finn, the highest-ranking Catholic official ever to face U.S. criminal charges in a child sexual abuse case, and the diocese both pleaded not guilty in October. The cover-up charge carries a penalty of up to one year in jail and a $1,000 fine.

Missouri law requires clergy, teachers and others in positions of authority involving children to report suspected child abuse. A grand jury in October charged Finn and the diocese for failing to abide by the mandatory reporting law.

But in motions filed late on Wednesday, Finn's attorneys argued that under the law a religious organization designates someone to report child abuse, and Finn was not one of those people. The responsibility instead belonged to a response team created by the diocese to review abuse complaints, they argued.

"Bishop Finn was not the diocesan designated reporter," the court filing said. "As such, Bishop Finn had no statutory duty to report."

No member of the diocese's response team is charged with knowing about Ratigan's pornography, which was discovered on his laptop by a computer technician who informed a church.

Ratigan was arrested in spring 2011 and accused of possessing hundreds of images of naked children and to have taken sexually explicit photographs since 2005 of at least five young girls, aged 2 to 12 years.

Finn has headed the diocese since 2005.

Jackson County Prosecutor Jean Peters Baker will oppose the defense motion, which was not a surprise, and has 15 days to respond, the prosecutor's spokesman, Michael Mansur, said on Thursday.

In addition to seeking dismissal of the case, Finn is asking that he be tried separately from the diocese.

Over the past decade, the Catholic church in the United States has faced numerous lawsuits demanding compensation for abuses by clergy, mostly involving sexual molestation of boys by priests. The suits frequently accuse the church hierarchy with covering up abuses and transferring offending priests to unsuspecting parishes.

The church has paid out more than $2 billion in settlements to victims and taken steps such as creating protocols for officials on how to respond to abuse charges. But critics have charged the church's response is inadequate, and that the abuses continue.

(Editing by Andrew Stern and Paul Thomasch)