Mr Fujii, say people who have spoken to him, was fed up with the power wielded over Mr Hatoyama by Ichiro Ozawa, secretary-general of the ruling Democratic Party of Japan (DPJ).
Support for the government has tumbled because of financial scandals surrounding Yukio Hatoyama, the prime minister, and Ichiro Ozawa, the DPJ's secretary-general.
Yukio Hatoyama, the prime minister, and his most influential backer, Ichiro Ozawa, have since been caught up in election-funding scandals that have shredded the government's credibility.