The late Pope John Paul II wrote letters to his close friend, the philosopher Anna-Teresa Tymieniecka, from Rome in order to avoid their being intercepted by the secret police in Poland.
The latest revelations about their relationship concern autumn in 1974, when Karol Józef Wojtyła was still Archbishop of Krakow in Poland and spent more than a month at a synod of bishops in Rome.
He wrote to her that he had not anwered before because he did not trust the post office in Poland but he had kept them and was re-reading them. "They are so meaningful and deeply personal, even if they are written in a philosophical 'code'," he wrote. At the end he said: "Finally there are issues which are too difficult for me to write about."
Although there is no suggestion that the future saint broke his vow of celibacy during the 32-year friendship, there is speculation that she fell in love with him and he struggled with his feelings for her but saw her as a gift from God and the friendship as a vocation.
Writing in The Telegraph, BBC journalist Ed Stourton said: "Cardinal Wojtyla gave Anna-Teresa what is known as a scapular, an item of devotional clothing which is worn next to the skin."
In one letter, the then-Cardinal told her: "Once – I remember exactly when and where – I heard these words 'I belong to you', and for me, first of all, the gift of a person resonated in them. I was afraid of this gift, but I knew from the beginning, and I know still better and better now, that I have to accept this gift as a gift from heaven."
The friendship began in 1973 whenand Polish-born Anna-Teresa Tymieniecka got in touch with him about her book on philosophy.
She had moved to the United States after the war, married and had three children. She kept every one of the 350 letters they exchanged over 30 years and in 2008 she sold them to the National Library of Poland in Warsaw. Pope John Paul II's letters have been translated and were read by Ed Stourton for the BBC but Stourton was not given access to the letters sent to the Pope by Tymieniecka, who died in 2014.
According to the BBC, Tymieniecka may have confronted the future pope about her feelings for him in Vermont, when he stayed with her and her family during a visit to the US with some Polish bishops.
In one letter he confessed he was struggling with their relationship. Describing her as a gift from God, he wrote: "If I did not have this conviction, some moral certainty of Grace, and of acting in obedience to it, I would not dare act like this." Later, he he wrote: "My dear Teresa, I have received all three letters. You write about being torn apart, but I could find no answer to these words."
There was a brief period of estrangement when the correspondence was limited to seasonal exchanges of cards, after he became Pope. She felt betrayed because Vatican officials did not want to acknowledge her role in his life.
But she was one of the only people allowed to see him in hospital after the assassination attempt in 1981. "I am overwhelmed by sadness and anxiety, and want desperately to be close to you," she telegrammed. "I arrive on Saturday."
Stourton reveals that the professor became a source of emotional support to Pope John Paul II during his long illness when he suffered from Parkinson's.
Stourton also wrote for the BBC: "It is public knowledge that for four years Cardinal Wojtyla and Anna-Teresa Tymieniecka collaborated on an English-language version of a book on philosophy he wrote while teaching at Lublin University, and Anna-Teresa Tymieniecka has a walk-on part in several John Paul biographies. But the relationship was much deeper and more complex, and continued for far longer than has previously been recognised."
Tymieniecka's husband, Hendrik Houthakker, was an economist at Harvard and after the collapse of communism advised the Pope on post-communist economies. Pope John Paul II granted him a papal knighthood in recognition of his services.
Panorama: The Secret Letters of Pope John Paul II is available to watch on demand.