Singh (65) breathed his last in Appolo hospital in Delhi where he was undergoing treatment for long.
He is survived by wife and three sons.
In his condolence message, the CM described the departed leader as a great socialist leader whose death has created a void in the social and political field.
Singh had represented Madhuban Assembly constituency in East Champaran four times on RJD ticket from 1985 to 2004.
Singh served as minister in the state government of both Lalu Prasad and his wife Rabri Devi.
He was elected Lok Sabha member from Sheohar on RJD ticket from 2004-9.
RJD President Lalu Prasad and Rabri Devi led the party in condoling the death of party leader. The party flag was lowered in the state headquarters in respect of the departed leader, RJD state spokesman Randhir Yadav said.
The body of the leader would be brought here tomorrow and his final rites would be performed in his native Banjaria village in East Champaran on January 13.