Pope John Paul II had an intimate 32-year relationship with a Polish philosopher who is believed to have once declared her love to him while he was still a cardinal, according to letters discovered by the BBC.

There is no indication that the former pope, who died in 2005 and was made a saint by Pope Francis, had a sexual relationship with Anna-Teresa Tymieniecka or ever broke his vow of celibacy, but the correspondence sheds light on the depth of the relationship Pope John Paul II had with Tymieniecka when he was archbishop of Kraków, including the possibility that he struggled to come to terms with his feelings for her.

According to letters unearthed by the BBC, which is airing a programme about the relationship on Monday night on BBC1, their friendship began in 1973, when Tymieniecka contacted Karol Wojtyła, the future John Paul II, about a book she had written on philosophy. The two had much in common: they had both been born in Poland and survived the Nazi occupation during the second world war. Tymieniecka moved to the US after the war, where she married and had three children and had an academic career as a philosopher.

According to the BBC, the pair met on several occasions after becoming friends in order to work on an updated version of the then cardinal’s book, The Acting Person. At one point in 1974, he told her that he was re-reading four of her letters because they were “so meaningful and deeply personal”.

When Cardinal Wojtyła was asked to lead a Polish delegation of bishops to a Catholic meeting in the US, Tymieniecka invited him to stay at her family’s house just outside of Pomfret, Vermont. “It was just the sort of outdoor life he enjoyed and photographs that I think were taken at the time show him at his most relaxed,” wrote Ed Stourton, of the BBC.

The BBC contends that Tymieniecka may have confronted the future pope about her feelings for him in Vermont, because he later wrote a letter to her confessing that he was struggling with their relationship. After telling her he believed she was 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.”

In one letter after that period, 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.”

Facebook Twitter Pinterest Pope John Paul II and Anna-Teresa Tymieniecka. Photograph: provided by Bill and Jadwiga Smith

The letters from the pope at the heart of the BBC story were bought by the National Library of Poland in 2008. While such letters would usually have been put on display, these were not open for public viewing, the BBC said, and it saw the letters exclusively.

But only one part of the puzzle has been revealed: the BBC said it had not seen any of Tymieniecka’s letters and that the National Library of Poland had not confirmed that it had any of her correspondence.

The fact that a pope would have a close relationship with a woman – including a married woman – is not entirely surprising to papal experts. But Jimmy Burns, who has written a biography of the current pope, Pope Francis, said it did humanise the former pope. “If he managed to have a good, strong emotional relationship, without breaking his vows, it humanises him. From an ethical point of view, one can’t hold anything against him,” Burns said.

Facebook Twitter Pinterest Pope John Paul II and Anna-Teresa Tymieniecka. Photograph: provided by Bill and Jadwiga Smith

The former pope is viewed as one of the most important figures of the 20th century, in part for his stance against communism. Reflecting on the importance of priestly celibacy, Pope John Paul II once wrote that it expressed “the church’s will” that “the church, as the spouse of Jesus Christ, wishes to be loved by the priest in the total and exclusive manner in which Jesus Christ … loved her”.

The Vatican said in response to the BBC programme that it was already known that Pope John Paul II “was friendly” with Tymieniecka.