He was 86.
Pillai is survived by his wife, four sons and a daughter, family sources said.
He was twice elected to the Assembly from Eravipuram in Kollam district in 1987 and 1996.
He was the labour and irrigation minister in the LDF ministry headed by the late E K Nayanar in 1996.
Pillai, a trade union activist, was RSP state secretary between 2008 and 2012.
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He also served as director of Kerala Co-operative Bank, Kerala State Warehousing Corporation and and Kerala State Coir Marketing Federation.
The Chief Minister, in his condolence message, said Pillai's contribution as Labour and Irrigation Minister in the Nayanar ministry was significant.
He said Pillai was a leader who stood for Left unity.
His firm stand was that RSP should not be with right parties, Vijayan said, apparently referring to RSP in the new leadership moving to Congress-led UDF in Kerala.
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