Veronica Ralston claims to have ordered a DNA test to put to rest rumours over the Academy Award winner's paternity and she alleges the results showed he was not biologically related to his mother Karin or father Erik.
Ralston, who has published her shocking claims in a new book called The Love Child and the Changeling, is convinced Karin Bergman's baby died during childbirth and another infant was brought up in place of their dead son.
Ralston reveals, "I contacted the (Swedish) board of forensic medicine to see if it was possible to perform a DNA analysis to clarify this. I suggested that they could investigate stamps which Ingmar Bergman had licked and sent on letters and postcards to his parents and compare them with my DNA.
"When my grandmother Karin Bergman gave birth to her son on July 14, 1918, she had been very sick for a long period of time, and it's possible the baby didn't survive. I think that... her husband Erik then switched the child with (another) baby."
Bergman died in 2007 aged 89.