LONDON — Archbishop of Canterbury Justin Welby, the spiritual leader of the global Anglican church, resigned Tuesday after an investigation found that he failed to promptly report serial physical and sexual abuse by a volunteer at Christian summer camps.

Welby said in a statement that he was stepping down “in sorrow” and “having sought the gracious permission of His Majesty The King.”

He had faced mounting calls to resign since the report was published last Thursday.

A petition calling for…