Michelle 'Bombshell' McGee said she had a months-long affair with Sandra Bullock's husband Jesse James while Bullock was filming the movie that won her an Oscar.
McGee told In Touch magazine she and James had sex repeatedly while Bullock was shooting "The Blind Side" in 2009.
She told the magazine she thought James and Bullock were separated.
"I would never have hooked up with him if I thought he was a married man," she said.
Following the revelations, Bullock issued a statement saying she was canceling her appearance at the London premiere of "The Blind Side."
"Due to unforeseen personal reasons a trip abroad to support 'The Blind Side' has been deemed impossible at this time," Bullock said in a statement. "I apologize for any inconvenience this may have caused and thank you for your continued support of the film."
She moved out of the home she shares with her husband of six years on Monday, People magazine reported.
James also deleted his Twitter account Wednesday. He had recently used his account to publicize his lost dog, Cinnabun, who was then found.
Bullock won the Academy Award for Best Actress for her role in 'The Blind Side.' In an interview with Barbara Walters before the Oscars ceremony, she said of James, "I’d never before met a man who I felt had my back."
But McGee said that while Bullock was in Atlanta working on the movie, she had sex at least once a week with the "Monster Garage" star.
McGee said she originally reached out to James because she wanted to get a modeling job at his company West Coast Choppers. She said she was surprised when he responded and asked her to send photos to his personal e-mail, In Touch reports.
She said she drove to meet him at his office, where she asked him about Bullock.
"He said, 'She doesn't live here. She has a house in Austin. She is filming, and I can't talk about it,'" McGee told the magazine.
She said they had sex several times in the weeks that followed.
Before marrying Sandra Bullock in 2005, Jesse James was married to porn star Janine Lindemulder, with whom he is now in a custody battle over their young daughter.