(CNN) A new report from Germany's Catholic Church admits to "at least" 3,677 cases of child sex abuse by the clergy between 1946 and 2014, according to local publications Die Zeit and Spiegel Online who have seen a leaked copy.

The report -- commissioned by the German Bishops' Conference and conducted by the Universities of Giessen, Heidelberg and Mannheim -- has taken four years to assemble and will be released in full on September 25.

According to both media outlets, the study says the victims were mostly boys, more than half of whom were aged 13 or younger. Every sixth case involved a rape and at least 1,670 clergy were involved, the report also reveals.

"We know the extent of the sexual abuse that has been demonstrated by the study. We are dismayed and ashamed by it," Bishop Stephan Ackermann said in a written statement to CNN responding to reports of the leak.

Ackermann said in the statement that the aim of the study was to "gain more clarity and transparency about this dark side of our church, for the sake of those affected, but also to see for ourselves the wrongdoings and to ensure this cannot be repeated."

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