Now Fr Gelli has spoken of how she stopped him on the way to the gym to ask about marriage - he is convinced that the Princess was seeking Christian approval to marry a Muslim man.
"She wanted to know if it was possible for two people of different religions to marry. I told her it was," Fr Gelli told The Sunday Express.
"As we spoke her telephone rang. It was obviously Dodi. Her eyes lit up. As she was leaving she asked me if I would be able to perform the service when she got married. Her love was obvious."
As she cruised the Mediterranean on Mohamed Fayed's yacht, the Princess phoned Fr Gelli to say she had good news.
The priest was asked to meet with her at Kensington Pace after the holiday, but she and Dodi would die just days later in the horror crash.
"I feel sure that if Diana and Dodi had not been killed, they would have married," added Italian-born Fr Gelli.
He said that Diana was happy, in love with Dodi and looking forward to the future. He first spoke out three years after Princess Diana's death, but said that he was not betraying her trust.
The Prince and Princess of Wales on their wedding day in July, 1981. Photo / Getty
He believed that the Princess marrying at his church would cause tension within the Church of England, which would have to face saying whether Dodi, a follower of Islam, could marry there.
Fr Gelli was also popular among businessmen and politicians during his eight years in Kensington.
The Princess' said she liked the priest because he was outgoing, confident and had an ability to listen.
And even though Diana asked that he call her by her name, Fr Gelli insisted on calling her "Your Highness".
He said that she was keen to know his impression on Muslim lifestyle, especially in how women are treated in Islamic Society.
Fr Gelli said she was impressed with the respect she received from Dodi and she believed he could offer her love and security.
At on service to mark the anniversary of her death, held outside Kensington Palace, he said: "Princess Diana was a wonderful, caring philanthropist.
"She would come sometimes into the church and sit at the back and pray."
Father Gelli urged people to pray not only for the couple, but also Princes William and Harry.
The priest, who grew up in Rome and holds degrees in theology, education and Islamic studies, plans to hold a service later this month but has not decided on a location.
