Sandra Bullock's Oscar celebrations have come to a crashing halt thanks to new tabloid allegations suggesting her husband Jesse James has been cheating on her.

Tattoo model Michelle 'Bombshell' McGee claims she enjoyed an 11-month affair with the TV mechanic while Bullock was on location shooting The Blind Side - the film that earned her an Academy Award at the beginning of the month.

McGee tells In Touch magazine that James told her that he and the actress had split, stating, "I would never have hooked up with him if I thought he was a married man. He gave me the impression they were separated."

The model tells the publication her relationship with the Monster Garage star was intimate and highly charged, and she reveals she called James by a pet name, Vanilla Gorilla - because he was so "well-endowed".

She claims she first met James when she approached him about a possible modeling job, and they started exchanging information via email.

Then he invited her to meet him at his Orange County headquarters, gave her a tour of his monster garage, and then invited her back into his office.

McGee recalls, "He wanted to watch movies, but I asked him, 'What's going on with you and Sandra?' He said, 'She doesn't live here. She has a house in Austin. She is filming, and I can't talk about it.'"

From that, McGee assumed the two had split and she embarked on a romance with the reality TV star.

The scandal has hit Bullock hard - she has pulled out of attending the British premiere of The Blind Side in London, prompting film bosses to scrap the event.

The red carpet screening of the hit movie was due to take place next Tuesday.

A spokeswoman for an agency handling publicity for the movie tells WENN the decision to cancel was made after Bullock informed organizers she was no longer available to travel across the Atlantic for the event.

She says, "Because Sandra isn't coming any more, there isn't going to be an actual premiere. She isn't coming over. I don't know the reason why. We just had the message relayed to us this morning."

Bullock's representative was unavailable for comment.

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Sandra Bullock Pictures