The 'Rosemary's Baby' actress - who has four biological and five adopted kids from her marriages to composer Andre Previn and director Woody Allen - admits the crooner, who she was married for less than two years between 1966 and 1968, was the great love of her life and she continued sleeping with him long after they separated.
She admitted: ''We never really split up.''
Asked if Ronan, her only biological child with the filmmaker, could be Sinatra's child, she told Vanity Fair magazine: ''Possibly.''
Although no DNA testing has ever been done on Ronan, now 25, he is treated as a member of the Sinatra family.
Nancy Sinatra Jr., the daughter of the singer - who died aged 82 in 1998 - told the publication in an email: ''He is a big part of us, and we are blessed to have him in our lives.''
In 1992, Mia accused Woody of molesting their daughter Dylan then seven, and sought Sinatra's help in protecting her because she was concerned the filmmaker's alleged connections to criminals in the Teamsters organisation.
The 'My Way' star set up a meeting for her with a Mafioso pal with whom she met in the back of a car, where he told her: ''The Teamsters? Don't worry about it. We own the Teamsters.''
Mia, 68, recalled: ''I remember babbling, 'Thank you, thank you'. Off he went, and I did feel safer.''
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