Announcing his decision at a press conference, he came out with an explanation that his vote did not lead to the fall of Atal Bihari Vajpayee Government in 1999.
"I have sent my resignation letter to AICC president Sonia Gandhi. Since the time of Vajpayee government's fall due to one vote in 1999, I have been humiliated. Though I voted against Vajpayee government in wake of a whip, the party did not come to my rescue at any time," Gamang told reporters.
Gamang, who was elected from Odisha's Koraput Lok Sabha segment eight times in a row from the 5th Lok Sabha in 1972, said he was again elected to the lower house of Parliament from the same seat in 2004 for the ninth time.
The Congress veteran had also been a Union Minister under the Prime Ministership of Indira Gandhi for one year and five years each under Rajiv Gandhi and P V Narasihma Rao.
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"It was not my vote which turned the tables against Vajpayee government but cross-voting of Prof Saifuddin Soz. Vajpayee government lost the confidence motion by one vote for the cross voting of Prof Soz as he had gone against the whip of his party and voted against the motion," Gamang said.
However, the 1999 no-confidence motion against the then Vajpayee government became a turning point in his political life, Gamang said.
Asked whether he will join any political party, he called himself a "liability".
"Why will any political party take me? I have been accused of defeating the Vajpayee government. Therefore, BJP may not accept me. There is no question of joining any regional party (BJD)," he said.
Gamang said he was being seen as the "bad" element due to a misconception that his vote ensured the defeat of Vajpayee government.
His resignation from Congress coupled with his explanation about April 17, 1999 trust vote in Lok Sabha assumed significance as the move came barely a month after the veteran leader had a meeting with BJP leader and Union Tribal Affairs Minister Jual Oram in Delhi.