The Valentino fashion house had even released a press statement in advance of the Oscars disclosing that it would be dressing the Les Miserables actress in addition to Sally Field andJennifer Aniston.
Anne has since issued an apology to the designer, saying: "It came to my attention late Saturday night that there would be a dress worn to the Oscars that is remarkably similar to the Valentino I had intended to wear."
Anne seemed to be referring to her Les Miserables co-star Amanda Seyfried's grey Alexander McQueen gown. "I decided it was best for all involved to change my plans," she explained.
"Though I love the dress I did wear, it was a difficult last-minute decision," added the 30-year-old, who picked up an Academy Award for Best Supporting Actress. She contacted her stylist Rachel Zoe to help with a selection of back-up dresses.
On the night itself Anne explained that it was a last-minute decision. "I didn't even know what I was wearing until about three hours ago," she said.
Valentino, who is responsible for the bold red number heralded by fashion fans at Anne's Oscars appearance in 2011, also created the actress' bespoke wedding gown which she wore when she married actor Adam Schulman in September 2012.