Lord Hattersley told the tale of their romance in his new book The Catholics, a study of the Catholic Church in Britain since the Reformation.
Lord Hattersley wrote that his father was the parish priest of St Joseph's Church in Nottingham and had met his mother after he agreed to instruct her for admission into the Catholic Church ahead of her marriage.
"Father Hattersley had performed the wedding ceremony," the former MP revealed in the book.
"Two weeks later the priest and the bride ran away together. For the next 45 years they lived in bliss - married after my mother's first husband died in 1956."
Fr Hattersley was ex-communicated from the Church over the scandal and died an atheist in 1973.
Lord Hattersley discovered the truth about the scandal when he met the Bishop of Nottingham while he was researching a novel based on his father's life story.
Lord Hattersley told the Telegraph: "My father and I were very close. It was a very hard act for him to walk away from the church and I was very proud of him for doing that."