His charge: despite an injunction on Mr Roy from a court in north Bengal's Alipurduar, the BJP leader was repeatedly claiming Mr Banerjee and not the Trinamool, nor the West Bengal government, were owners of the two most prominent logos in the state - Biswa Bangla and Jago Bangla.
"Mukul Roy says that I am the owner of Biswa Bangla and Jago Bangla. But where is the proof? If he can prove it, I will quit politics. If he can't, he should leave politics," Mr Banerjee said.
After Mr Banerjee filed a civil defamation case against Mr Roy at Alipurduar on November 21, Mr Roy filed a criminal defamation case against Mr Banerjee at Patiala House court in Delhi. Now Mr Banerjee has filed a criminal defamation suit too.
Ever since he switched to the BJP, Mr Roy has attacked Abhishek Banerjee, someone extremely close to chief minister Mamata Banerjee.
On November 10, at his first public rally in Kolkata as a BJP leader, Mr Roy claimed Biswa Bangla, a widely used logo by the government, is not a state entity but a private limited company owned by Abhishek Banerjee.
The trademark ownership of Jago Bangla, the name of Trinamool's newspaper, the party's popular slogan - Ma Mati Manush - and even the Trinamool symbol are in dispute, according to Mr Roy.
The rivalry between Mr Roy and Mr Banerjee goes back to early 2015. The two men - both close to Mamata Banerjee - fell out after Mr Roy felt Abhishek Banerjee was replacing his men in key posts. Also, after Mr Roy was quizzed in the Saradha chit fund case by the CBI, Ms Banerjee, say sources, had voiced suspicion that Mr Roy named her nephew during his interrogation.