"It was clear from his comments and writing that he was no longer in the BJP. The party gave him a lot of respect and important positions but his conduct has been improper.
"His comments sounded more like that of a Congress leader or somebody working at the behest of the opposition party," BJP spokesperson and national media head Anil Baluni said.
The party is not surprised by Mr Sinha's announcement, he said.
Mr Sinha, 80, today announced in Patna that he was quitting the BJP and would no longer be a part of party politics.
Mr Sinha held finance finance and external affairs portfolios in the Atal Bihari Vajpayee government but was sidelined after Narendra Modi and Amit Shah came at the helm.
In a newspaper article, Mr Sinha had criticised Arun Jaitley over what he called the mess the finance minister has made of the economy after which Mr Jaitley hit back calling him "a job applicant at 80 years" who has forgotten his record as finance minister.
In a no-holds barred attack, Mr Sinha had also said that demonetisation had proved to be an unmitigated economic disaster and that the Goods and Services Tax (GST) was badly conceived and poorly implemented. He had also deprecated the Centre's handling of Kashmir.
Everybody in the BJP is living in fear. We are not, Mr Sinha had said while "Rashtriya Manch" (National Forum) on January 30 as an "apolitical forum" to highlight the alleged"anti-people" policies of the Centre.
Mr Sinha today alleged that democracy was "under threat" under the present government and that he would launch a strong movement to "save democracy" in the country.
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