Bullock, who won her first Oscar last week for "The Blind Side," had been due in London to promote the drama about a woman who befriends a homeless black teenager.
But she issued a statement to film studio Warner Bros. on Wednesday saying that "due to unforeseen personal reasons a trip abroad to support 'The Blind Side' has been deemed impossible at this time.
"I apologize for any inconvenience this may have caused and thank you for your continued support of the film," her statement said.
The British premiere of the movie was due to take place in London on Tuesday. Warner Bros. declined to comment. The announcement came as In Touch magazine published an interview with a California model who claimed she had an affair with Bullock's husband of five years while the actress was away in Atlanta filming "The Blind Side" last year.
Bullock married custom motorcycle manufacturer and reality TV star Jesse James in 2005. In several award ceremonies earlier this year, the actress attributed much of her recent success to her happiness at home.
Bullock's publicist could not immediately be reached for comment on Wednesday.
If the claims of infidelity by James prove correct, Bullock will join a long list of actresses whose relationships have fallen apart shortly after they won an Oscar.