Kolkata: Mukul Roy, one of the founders of the Trinamool Congress and at one point the closest confidant of West Bengal chief minister Mamata Banerjee, on Monday announced that he will quit the party, capping months of speculation that the 63-year-old leader was looking to join the Bharatiya Janata Party (BJP).

For Banerjee, this is the most notable defection from her Trinamool Congress party which was founded in 1998 with Roy as the proposer for its formation. Roy, who until recently served as the Trinamool Congress’s national vice-president, was on Monday suspended from the party for six years, a couple of hours after he announced his decision to quit.

Addressing the media, Roy on Monday said he was resigning from the party’s working committee immediately and that after the Durga puja he will step down as a member of the Rajya Sabha. Severing ties with the party he had co-founded was painful and difficult, Roy said, but declined to clarify why he chose to distance himself from the Trinamool Congress.

Within a couple of hours, Partha Chatterjee, secretary general of the Trinamool Congress, said the party was closely observing Roy’s activities for some time. Bowing to pressure from central investigating agencies, Roy has been trying to inflict damage to the Trinamool Congress, and hence it had been decided that he is to be suspended from the party for six years, Chatterjee announced.

Within the Trinamool Congress, Roy was a key election strategist. In the past few years, he was embroiled in several controversies and was facing probe by the Central Bureau of Investigation (CBI) for allegedly helping the now defunct Saradha Group ply its illegitimate deposit collection business. Roy was also seen on the controversial Narada TV footage, and is one of the accused in the ongoing investigation started by central agencies against 13 Trinamool Congress leaders on charges of illegal gratification.

Roy’s departure could turn out to be a bigger gain for the BJP than a loss for the Trinamool Congress, said Biswanath Chakraborty, a professor of political science at Kolkata’s Rabindra Bharati University and an election analyst. The BJP in West Bengal does not immediately have an election strategist of Roy’s experience and understanding, he added.

However, it is not immediately clear whether he is going to join BJP. He himself declined to comment over the speculation, saying that he has been in touch with several political parties. His son, Subhranshu Roy, who is a Trinamool Congress legislator from the North 24 Parganas district, said in television interviews on Monday that he wasn’t planning to quit the party.

Rahul Sinha, a spokesperson for the BJP in West Bengal, said there has been no discussion yet over Roy joining the party.

Mukul Roy is likely going to launch his own political outfit, which eventually would align with the BJP, said a close aide, who asked not to be named. “Within the Trinamool Congress, he was on a slippery slope from the time CBI interrogated him in connection with the Saradha Group scam," this person said. Recently, Banerjee abolished the post of vice-president, reducing Roy to a working committee member only.

Roy, this person claimed, has the backing of several other Trinamool Congress leaders. “They may not immediately quit the party, but what is important here is a sizeable section of disgruntled leaders are now ready to walk out on Banerjee," he said.