LONDON—The Church of England “colluded” with and helped to hide the long-term sexual abuse of young men by one of its former bishops, the head of the church said Thursday.

Archbishop of Canterbury Justin Welby apologized to the victims who spoke out and helped bring ex-bishop Peter Ball to justice as the church published a detailed report into how it handled the case.

Welby ordered the report after Ball was convicted and imprisoned in 2015 for misconduct in public office and indecent assaults against teenagers and young men over a period of 20 years. Ball, who admitted to abusing 18 people, was released after serving 16 months.

The report said Ball’s conduct “caused serious and enduring damage to the lives of many men,” and that “the church at its most senior levels and over many years supported him unwisely.”

Describing the report as “harrowing reading,” Welby said: “The church colluded and concealed rather than seeking to help those who were brave enough to come forward.”

“This is inexcusable and shocking behaviour,” he said, adding that while most of what happened took place years ago, “we can never be complacent, we must learn lessons.”