Montand, whose marriage to the actress Simone Signoret was for years considered unusually strong in the show business world, was exposed in a just-released book by Benjamin Castaldi, a French television presenter and the son of Catherine Allegret, Montand’s stepdaughter.
Details of the long-hidden affair were published yesterday in Paris Match.
Castaldi’s book, Now Everything Must Be Told, which has just hit French bookshops, recounts how Signoret and Ms Allegret were "destroyed" by the latter’s complex relationship with Montand.
Although Signoret reacted bravely in public to her husband’s widely known affair with Monroe, his co-star in the aptly titled 1960 Hollywood film Let’s Make Love, remarking "How many men do you know who could resist Marilyn Monroe?" she was privately devastated.
Castaldi recounts that it was from that moment that the legendary French beauty ceased to take care of her appearance.
Signoret’s meeting with Montand, a rising cabaret star and protege of Edith Piaf, had precipitated her divorce from her first husband, the film director Yves Allegret in 1949.
She married Montand in 1951 and he was a devoted father to her daughter Catherine, four. But Castaldi recounts that as his mother grew up into a young women with the captivating beauty of her mother, Montand’s affections turned from the purely paternal to those of a lover.
Unlike with infamous affair between Woody Allen and Soon-Yi Previn, the adopted daughter of his long-term partner Mia Farrow - which became a scandal in the 1990s when Farrow revealed details of it - Signoret and her daughter chose to remain silent about their family secret.
Signoret remained with Montand until her death in 1985.
Castaldi says he only discovered his mother’s true relationship with Montand in 2002, a few days after she stood him up at his marriage.
In a telephone call, Ms Allegret finally explained to her son the source of her own problems. "I understood at last the consequences which this affair had for my mother," he wrote.
"The weight of an enormous guilt had deformed her relationships with others. The weight of the secret which surrounded these relationships had the effect of amplifying her feeling of isolation."
Castaldi also reveals in his book how Montand, the star of The Wages of Fear and Jean de Florette, who died 13 years ago, could also be "brutal", "spiteful" and "obsessive", with "an odious attitude towards women".