Mother Teresa's hidden faith struggle is to be laid bare in a book that shows she felt alone and separated from God.

Mother Teresa's ordeals were first laid out to a series of confessors and confidants and became public knowledge in 2003 during the investigation into her cause for sainthood, a process fast-tracked by Pope John Paul II.

But Come Be My Light: The Private Writings of the 'Saint of Calcutta', to be released next Tuesday by Doubleday, collects her thoughts in one place for the first time, inviting a closer review of her life 10 years after her death.

The book was edited by the Rev Brian Kolodiejchuk, a priest who knew Mother Teresa for 20 years and is the postulator for her sainthood cause. It depicts her as a mystic who experienced visions of Jesus speaking to her early in her ministry, only to lose that connection and long for it for most of her last four decades.

"I have no Faith - I dare not utter the words & thoughts that crowd in my heart - & make me suffer untold agony," she wrote in an undated letter.

In 1956, she wrote: "Such deep longing for God and ... repulsed empty no faith no love no zeal ... Heaven means nothing pray for me please that I keep smiling at Him in spite of everything."

Rev Richard McBrien, author of Lives of the Saints, said her struggles made her more human. "It shows that she wasn't a plaster-of-paris saint who never had a doubt about God or the ultimate meaning of life," he said. Mother Teresa was beatified in 2003. Under Catholic tradition, an additional miracle attributable to her must be verified before sainthood.