The poll panel asked the 65-year-old leader to "be careful not to lure voters with new schemes," sources said.
The Election Commission alleged that the Chief Minister made announcements for new schemes by the state government while campaigning for her party, AIADMK's candidate on November 28 in Yercaud, where bypoll was held on Wednesday.
In her reply to the poll panel on Tuesday, Ms Jayalalithaa had denied having violated the model code. "I made a general statement that whatever is required for the uplift of the people in the area would be provided," she wrote.
"My speeches to the electorate were in Tamil. I never made any promise or announcement of any new schemes in any of my speeches on November 28," she had written in her reply to the poll panel.
"I have not violated the code. Please drop the proposed action against me," she is reported to have written to the Election Commission.
The Yercaud election, the fourth by-poll in Tamil Nadu since 2011 after Ms Jayalalithaa stormed to power, was necessitated following the death of sitting AIADMK legislator C Perumal earlier this year.