The researchers analysing his letters believe that in the early stage of their relationship, around 1975, Anna-Teresa Tymieniecka told him she was in love with him.

The former Pope, who died in 2005 and was canonised in 2014, met Anna-Teresa Tymieniecka before he was elected Pope in 1978.

The letters, which lay undiscovered and away from public view in the Polish national library until they were shown to the BBC's Panorama programme, tell of a complex and intense friendship in which it has been suggested that Ms Tymieniecka told the pontiff she loved him.

A person once asked me, in a provocative manner, if I approved of homosexuality.

I replied with another question: ‘Tell me: when God looks at a gay person, does he endorse the existence of this person with love, or reject and condemn this person? Here we enter into the mystery of the human being.2.

” Two years ago Tuesday, Pope Francis uttered these words, sending shockwaves throughout the Catholic Church and the world.

His position stood in contrast to that of his predecessors: Months earlier, Pope Benedict XVI suggested that gay marriage was a threat to global peace.

"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.

"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." Even after he became Pope, the two continued their correspondence and she visited the Vatican on various occasions.

Carl Bernstein, the American journalist famous for uncovering the Watergate scandal was the first person to write about the significance of the relationship in the 1990s.