POPE St John Paul II had a close relationship with a married woman, letters sent by the former pontiff have shown.
There is no suggestion he broke his vow of celibacy, but the tone of some of the correspondence point to strong feelings between the former Pope and Polish-born philosopher and writer Anna-Teresa Tymieniecka.
The letters are the subject of a BBC Panorama documentary due to be broadcast tonight.
The documentary only examines St John Paul II's letters as it does not have access to those written by Ms Tymienicka, who died in 2014.
Edward Stourton, the journalist who made the documentary, said more than 350 letters were found at the National Library of Poland, the first dated in 1973 and the last a few months before his death in 2005.
"I would say they were more than friends but less than lovers," he said.
"One of the fascinating stories that comes out of these letters is of a struggle to contain what was certainly a very intense relationship which mixed emotions and philosophical ideas in proper Christian boundaries."
The friendship began in 1973 when Ms Tymieniecka contacted Cardinal Karol Wojtyla, then Archbishop of Krakow, about a book on philosophy that he had written.
In 1974, Cardinal Wojtyla wrote that he was re-reading four of her letters because they were "so meaningful and deeply personal."
The two spent camping and skiing holidays together and went on country walks.
In September 1976, he called her a "gift from God".
"My dear Teresa. You write about being torn apart, but I could find no answer to these words."
In another letter in the same month the former Pope wrote: "Already last year I was looking for an answer to these words, 'I belong to you', and finally, before leaving Poland, I found a way - a scapular."
A scapular is a piece of cloth worn as part of the habit of monastic orders and the then Cardinal Karol Wojtyla gave Ms Tymieniecka his.
"The dimension in which I accept and feel you everywhere in all kinds of situations, when you are close, and when you are far away," he added.
It is believed copies of Ms Tymieniecka were included in an archive that was sold to the Polish National Library in 2008.
The Polish National Library has not confirmed whether it holds the letters.
Marsha Malinowski, a rare manuscripts dealer who negotiated the sale of the letters, said she believed Ms Tymieniecka fell in love with Cardinal Wojtyla in the early days of their relationship.
"I think that it's completely reflected in the correspondence," she told the BBC.
Ms Tymieniecka had three children with her husband, Hendrik Houthakker, who was a distinguished Harvard economist.
He advised St John Paul II on post-communist economies, and the Pope granted him a papal knighthood in recognition of his services.
