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Veerendra Kumar, the chairman and managing director of the Malayalam daily newspaper Mathrubhumi, is also a director in PTI.
Kumar was elected to the Rajya Sabha in March 2016.
"I have already communicated the decision to Mr Nitish Kumar", he said.
Asked when he will put in his papers, Kumar said 'I will let you (media) know'.
On his plans, Kumar said: "First we will discuss the party's future in the state committee and decide further course of action."
He said the Bihar Chief Minister told him not to resign and that he considers him as one of the tall socialist leaders in the country.
"But I have conveyed my decision to Mr Nitish Kumar and party", he said.
However, Kumar said he had held talks with some of the leaders of JD(S), a partner in the LDF, in the state and that they expressed desire to work unitedly.
Kumar, who was the leader of the Socialist Janata (Democratic) party, merged it with the Janata Dal (United) in 2014.
He was a minister in the United Front government after being elected to the Lok Sabha in 1996 on the Janata Dal ticket.
He was in Janata Dal (Secular) headed by former Prime Minister H D Deve Gowda from 1999 to 2010.