Roy quit the Banerjee-led party on September 25 and resigned as a Rajya Sabha member on October 11. He said that it will be his "proud privilege" to work under Prime Minister Narendra Modi and Amit Shah.
He described the BJP as a secular party saying the TMC had alliance with the BJP earlier and it could not have established itself without BJP's support.
Roy, who was long the second most powerful leader of the TMC, claimed that people of West Bengal wanted an alternative to Chief Minister Mamata Banerjee, and will vote the BJP to power in the next polls.
The former railway minister was earlier also questioned by the CBI in connection with the multi-crore Saradha chit fund scam. "The BJP's aim is to use Roy, an insider until now, to engineer rifts within the Trinamool Congress", he added, asking not to be named.
Riding high on its performance in recent elections and a jump in its membership in Bengal, the BJP aims at increasing its Lok Sabha tally in the eastern state.
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Roy, a founding member of Trinamool Congress, was in-charge of the TMC organisation. West Bengal is among the states where the BJP is eyeing around 150 seats.
Incidentally, till a few months ago, the BJP had repeatedly launched verbal attacks against Mr Roy and other TMC leaders for their alleged involvement in corruption and scams.
"Mukul Roy has joined the BJP as an agent of Mamata Banerjee".
Known for his composure, shrewdness and organisational ability, Roy purportedly called the shots with regard to promotions and postings of key administrative and police officers after the TMC came to power in West Bengal in 2011. He stressed that the former TMC leader had expressed his desire to join the BJP "without any conditions".
JP national general secretary and in charge of West Bengal Kailash Vijayvargiya were also present on the occasion.
"This proves that a rot has set in BJP", says West Bengal Pradesh Congress chief Adhir Chowdhury.