The director had previously said that Bruni would be "wonderful" in his movie and later reports suggested that the pair had signed a deal.
However, Allen has now admitted that he is unsure whether he can risk hiring Bruni, who is married to French President Nicolas Sarkozy.
"Carla Bruni is not a woman who earns her living as an actress," he said on Swiss television. "She is a first lady."
He added: "She could be taken away at any moment by other duties, like a political crisis or an important event. I have to take account of the risks that there could be, and hiring her is far from certain."
Rachel McAdams is reportedly in talks to join the project alongside Owen Wilson and Marion Cotillard.