Hathaway became engaged to boyfriend Adam Shulman last year, and was aiming for a summer wedding after a lavish engagement party in New York.
The actress has been busy with work on the upcoming Batman movie The Dark Knight Rises, in which she plays Catwoman.
She is currently filming Les Miserables with stars Hugh Jackman and Amanda Seyfried.
A source told the Daily Mail: "Anne's worried there won't be time to get back in shape and grow her hair for a summer wedding. She also has to promote her movie Batman. They are thinking about postponing until later this year or maybe 2013."
For her role as Fantine in Les Miserables, Hathaway has lost weight and had her hair cut short to better portray the dying character.
Watch Anne Hathaway as Catwoman in the trailer for The Dark Knight Rises below: