Christie's expects to fetch up to €90,000 for the collection of love letters that Edith Piaf wrote to the French cyclist Louis Gérardin.

It is said that Piaf compressed more drama into her 48 years than most people could accommodate in multiple lifetimes. Now, a collection of previously unseen love letters that were written to the cyclist Louis Gérardin have added an extra lifetime's worth of passion to the singer of 'La Vie en Rose'.

The 54 letters, written between 1951 and 1952, shed light on Piaf's little-known relationship with the world championship cyclist, whom she called Toto, and trace her growing affection towards her "blue and blond angel".

Patricia de Fougerolle, a specialist in the department of books and manuscripts at Christie's auction house, said: "It's an exceptional collection of letters, a passionate correspondence which has never been seen before. They come from a time in Piaf's life of which, until now, we knew very little."

The correspondence starts two years after the death of the boxer Marcel Cerdan, widely believed to be the "true love of Piaf's life", and continued until one month before she married the French singer Jacques Pills.

While the life of the iconic Parisian street singer – who was discovered by a nightclub owner and given the stage name Piaf (or sparrow) – has become almost as famous as her work, the period in which she was still mourning the death of Cerdan while falling in love with Gérardin has remained private.

In one of the first letters, dated January 1952, Piaf writes: "My blue love, our first separation... darling, I think I can say that never has a man taken me as much, and I believe I'm making love for the first time."

Gérardin himself said that: "Forty-eight hours with Piaf are more tiring than a lap in the Tour de France". Piaf biographer Jean-Dominique Brierre said of the singer: "She fell in love easily, and was more in love with the idea of love than with a man himself. It was like falling in love for the first time.

"These letters come from a time that was so bad in Piaf's life. She pulled back from public life after she lost Marcel Cerdan. He was so significant to her because he died in an aeroplane crash before their love had a chance to die, and she cherished his memories. We can never know if their love would have lasted had he remained alive."

The singer became a national heroine in France and her death was marked by 40,000 mourners at Père Lachaise cemetery in Paris.

In the letters, she wrote about the changes that Gérardin brought into her life as well as her concerns over his jealous wife.

She promised to stop drinking and said that she wished to get married, settle in a house near Paris and become a perfect housewife. "My love, you have to help me to change. You will be my dear teacher and I'll listen wholeheartedly as if to a worshipped teacher," she wrote. The letters will be exhibited in Paris before being sold on 25 June.