Mother Teresa, who may be canonised as a saint by the Vatican later this year, had a deep crisis of faith in God for most part of her life, a set of her letters has revealed.

The correspondence, between Teresa and her confessors and superiors over a period of years, showed that she felt alone and in a state of spiritual pain from around 1949, roughly the time when she started taking care of the poor and dying in Kolkata.

The 'Saint of the Gutter' died on September 5, 1997, nine days after her 87th birthday.

Although she publicly proclaimed that her heart belonged "entirely to the Heart of Jesus", she wrote to the Rev Michael Van Der Peet, a spiritual confidant, in September 1979, that "Jesus has a very special love for you. As for me, the silence and emptiness is so great that I look and do not see, listen and do not hear. The tongue moves (in prayer) but does not speak."

The letter was written just a few weeks before she received the Nobel Peace Prize for her charitable work.

More than 40 other letters, many of which she had asked to be destroyed in her will, show her fighting off feelings of "darkness" and "torture" for the last nearly half-century of her life.

The letters are published for the first time in a new book titled Mother Teresa: Come Be My Light, edited by the Rev Brian Kolodiejchuk, a close friend.

He wrote that during that period, Mother Teresa did not feel God "in her heart or in the Eucharist."

Kolodiejchuk gathered the letters as part of the process to make Mother Teresa a saint, and is responsible for arguing in her favour.

He noted the letters would show people another side of her life, and said that the fact that she was able to continue her work during such torment was a sign of her spiritual heroism.

Mother Teresa has been beatified, and is awaiting canonisation, the process of declaring someone a saint.

The Catholic Association of Bengal has mounted a constant prayer for the last two weeks to push her cause forward in Rome.

The organisation has nominated 2007 as 'the year of Mother Teresa's Sainthood', since September 5 will mark the 10th anniversary of her death.

Euguen Gonsalves, the president of the association, said it was "clear that she lived a saint's life and there are no doubts for many in the world that she is already a saint."

However, the letters reveal that Mother Teresa, who was perpetually outwardly cheerful, struggled fiercely with her own faith.

She compared her problems to hell and at one point, had begun to doubt the existence of heaven and even God.

"The smile," she wrote, "is a mask or a cloak that covers everything. I spoke as if my very heart was in love with God, a tender personal love. If you were there you would have said, 'What hypocrisy'."