"We have sent a legal notice to Buddhadeb Bhattacharjee for using derogatory words against the Chief Minister and set a 48-hour deadline for him to apologise or face legal action," Trinamool Congress General Secretary Mukul Roy said.
Bhattacharjee had told a Bengali TV news channel in an interview on Tuesday that "I don't agree with the perception that she (Mamata Banerjee) is honest."
Leader of the Opposition Suryakanta Mishra of CPI(M) was also served a letter by Trinamool Congress for allegedly trying to malign the Chief Minister by questioning the sale of her paintings.
"Such further statements will force us to take legal action against him," the letter said. CPI(M) sources said TMC had issued the letter.