Singh breathed his last at a hospital around 11 AM, a CPI statement said.
He was 76. His wife predeceased him and he is survived by a son.
Chief Minister Nitish Kumar condoled the death and said he had great contribution to the fight for the rights of workers in the country.
The deceased CPI leader was a native of Alama village of Bihar's Nalanda district.
He started his political career in 1965-66 with the AISF, the student wing of the CPI.
He also served as president of the All India Trade Union Congress (AITUC).
The CPI statement said that his last rites would be performed in Patna tomorrow.
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