Suspended BJP leader Jaiprakash Majumdar, who had of late been critical of the saffron party state leadership's style of functioning, joined the TMC on Tuesday in the presence of Chief Minister Mamata Banerjee and other senior leaders.
He was appointed a state vice president of Bengal's ruling party.
The development came a day after Mr Majumdar held a meeting with BJP MP Locket Chatterjee, and said "the two share good terms and discussed politics among other issues".
At an organisational programme of the TMC during the day, senior leader Firhad Hakim handed over the party flag to Majumdar.
Welcoming him to the fold, the CM and TMC boss said, "Jaiprakash Majumdar will be the TMC state vice-president." Baptised in politics as a Congress activist, Majumdar had joined the saffron camp in 2014 after the grand old party's nationwide rout in the 2014 Lok Sabha polls.
"I want to be part of the developmental process that Mamata Banerjee initiated in West Bengal," he stated here after the switchover.
BJP state president Sukanta Majumdar, when approached, said Jaiprakash has been expelled.
"We don't want undisciplined soldiers in our party. Jaiprakash Majumdar has been expelled from the party," the state BJP chief said.
Mr Majumdar and Ritesh Tiwari were suspended from the BJP in January for anti-party activities.
The two senior BJP leaders were dropped from the new office bearers' committee last year.
Both were vice-presidents of the state unit in the previous committee. Mr Majumdar was also named one of the spokespersons in the saffron camp's new state committee announced a month back.
Mr Tiwari was, however, not included in the panel.
The BJP MP, who had held a meeting with Mr Jaiprakash and Mr Tiwari on Monday, said those loyal to the party would never desert it.
"Jaiprakash Majumdar had joined the BJP from the Congress. Now he has switched over to the TMC. Those loyal to a party would never switch camps. If I have any grievances, I will raise them within the party. That said, our party leaders should also see to it why its members are deserting the camp," Mr Chatterjee added.
Since the declaration of assembly poll results last year, five BJP MLAs, including the party's national vice president Mukul Roy, have crossed over to the TMC.
Last year, former union minister and senior BJP leader Babul Supriyo had joined the TMC, just months after he was dropped as a minister in the union ministry reshuffle.
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