Former Trinamul leader Tapas Roy joined the BJP on Wednesday and pledged to sign off his political career only from Narendra Modi's party while vouching to "fight" the alleged misrule of Bengal's ruling party.

Accepting the party flag from BJP state president Sukanta Majumdar, the five-time MLA said: “I am grateful that the BJP accepted me as one of the members of its family.... I commit to fulfilling all the duties and responsibilities that would be entrusted to me.”

Roy switching sides was expected after he put in his papers as Trinamul MLA on Monday and made clear that he would not remain with Mamata Banerjee's party. The veteran MLA had been critical of Trinamul's functioning and publicly criticised the party for not standing by him when the Enforcement Directorate conducted a raid on his central Calcutta residence in January in connection with a probe related to the alleged civic body recruitment scam.

Joining the saffron camp in the presence of leader of the Opposition in the Assembly Suvendu Adhikari and BJP Bengal minder Mangal Pandey, Roy attacked the Mamata government over "corruption" and the mishandling of the Sandeshkhali issue.

“This (Trinamul) party has become a party of demons. There is absolute lawlessness and mismanagement in the government. It has become a government of people like Sheikh Shahjahan, Uttam Sardar and Shibu Hazra. This is a government that doesn’t even bother to obey the judgments of (Calcutta) High Court and the Supreme Court,” he said after joining the BJP.

Several Trinamul leaders said that Roy was a leader of integrity and party should introspect the reasons that led the veteran to snap his ties with the party.

“I consider Tapas Roy to be my elder brother. I have known him since my college days. He left the party because of some personal grudge.... Bratya Basu and I attempted to dissuade him, but he had made up his mind.... If the party leadership had looked into his grievances at the right time, this situation would not have arisen. I will only request him not to allow the Suvendu virus to enter his body,” Trinamul leader Kunal Ghosh said.

The state BJP leadership is eyeing a major role for Roy in Calcutta. Adhikari said: "We firmly believe Tapas da will assist us in establishing a formidable organisational presence in north Calcutta."

Roy's resignation from the Assembly remains tied in technical knots. Speaker Biman Banerjee said Roy's resignation had not been accepted because of a "technical fault" and urged him to resubmit it.