The Democratic Indira Congress-Karunakaran (DIC-K) led by K Karunakaran would merge with Sharad Pawar's Nationalist Congress Party (NCP) on November 12.
The decision was announced at the NCP's general body meeting held in Kochi on Sunday.
The merger would take place at a public meeting in the presence of Pawar, Karunakaran and other leaders, according to NCP state president Cyriac John.
Reacting to the development, state secretary of the Communist Party of India Veliyam Bharghavan said if this happens the NCP would have to go out of the Left Democratic Front (LDF).
"You wait and see, the NCP here will virtually be controlled by Karunakaran henceforth and this is all but a ploy by him to make a strong return to the Congress," Bharghavan said at Kollam.
Karunakaran's son K Muraleedharan, in all probability, would be installed as the new state president of the NCP, informed sources said.
Karunakaran formed the DIC-K in May 2005 with an aim to end the Congress rule led by Oommen Chandy and to throw out the Congress-led United Democratic Front.
But after the Communist Party of India-Marxist said a firm "no" to Karunakaran's entry into the LDF in January, the veteran leader beat a hasty retreat and joined hands with the United Democratic Front and fought the April-May assembly polls together.
Here also Karunakaran was at the receiving end when he saw 17 out of his 18 candidates, including Muraleedharan, bite the dust.