Roy, who joined the Bharatiya Janata Party on November 3, on Friday had alleged that Banerjee, the nephew of West Bengal Chief Minister Mamata Banerjee, owned Biswa Bangla, a brand promoted by the state government.
Along with this Roy also claimed that the "Jago Bangla" newspaper, the mouthpiece of the TMC was also owned by Abhishek Banerjee.
"In a trade mark journal published by the Trade Marks authority of the Government of India, on May 8, 2017, the Biswa Bangla mark is shown as wholly owned by Abhishek Banerjee, the nephew of Mamata Banerjee, the Chief Minister of West Bengal", he said.
Abhishek even demanded an apology from Roy, who was considered the number-two man in the TMC for a long time.
The notice sent to the BJP headquarters in Kolkata stated that Roy should abstain from making and circulating derogatory statements.
In the defamation notice, Banerjee accused Roy of deliberately levying "baseless allegations" against him.
State's Additional Chief Secretary Rajiva Sinha had said: "The Biswa Bangla Marketing Corporation was incorporated in December 31, 2014 and its share holders are three government officials". He warned Roy of criminal and civil action if he failed to apologise.
Roy's counsel reportedly stated that they would counter the same "legally" thereby negating any chance of a "unconditional apology".
People of West Bengal can easily identify a political opportunist, said West Bengal Minister of state for Health Chandrima Bhattacharya.
Meanwhile, the Trinamool Congress organised a rally on Monday at Rani Rashmoni Avenue, the same venue where Mr. Roy had made these allegations.