NEW YORK (CNN/Money) - Martha Stewart, under investigation for possible securities law violations, resigned from the New York Stock Exchange board of directors Thursday just four months after being appointed.

NYSE Chairman and CEO Dick Grasso announced Stewart's resignation during a news conference Thursday. Grasso said Stewart resigned voluntarily and that she was not asked to leave. She turned in her resignation just before Thursday's NYSE board meeting and did not attend the meeting.

Richard Grasso, chairman and CEO of the New York Stock Exchange, comments on Martha Stewart's resignation from the NYSE board of directors. Play video (QuickTime, Real or Windows Media)

"We are saddened to lose Martha Stewart, who has built a brand and a company admired around the world," Grasso said in a statement. "Our board will miss Ms. Stewart's counsel and insight, and expressed its deep gratitude for her service."

The government is investigating whether Stewart had inside information when she sold her ImClone Systems Inc. stock just before its price plummeted.

"It was an honor and a privilege to serve on the NYSE board, and I appreciate Mr. Grasso's gracious expression of gratitude for my work," Stewart said in a statement late Thursday. "I did not want the media attention currently surrounding me to distract from the important work of the NYSE, and thus I felt it was appropriate to resign."

Stewart was nominated for the board seat March 22 and appointed in June. Her term was to expire in 2003, according to the NYSE Web site. She served on the Finance and Audit Committee.

Separately, Grasso said Thursday that Citigroup Inc.'s Michael Carpenter also resigned from the NYSE board. Carpenter's resignation last month as head of the Salomon Smith Barney unit made him ineligible to serve on the NYSE board. Carpenter's term was set to expire in 2003, according to the NYSE Web site.

Stewart's resignation comes a day after her broker's assistant at Merrill Lynch accepted a plea deal in which he has agreed to testify against her and her broker about the alleged insider trading, a development with the potential to blow a major hole in the home fashion guru's defense.

The assistant, 26-year-old Douglas Faneuil, pleaded guilty in U.S. Magistrate Court in New York Wednesday to taking bribes, including a plane ticket, extra pay and an extra week's vacation in exchange for his silence about the alleged trading.

Prosecutors are trying to prove that Stewart's broker, Peter Bacanovic, tipped her off last December that Samuel Waksal, a personal friend, was trying to sell his shares in biotech firm ImClone Systems Inc. (IMCL: up $0.04 to $7.25, Research, Estimates), which he founded. Stewart sold about 4,000 of her own shares in the company on the same day, according to documents she submitted to congressional investigators.

Waksal, who was indicted last month, pleaded not guilty to charges he tipped off family members that the Food and Drug Administration was about to reject ImClone's application for its cancer drug, Erbitux.

Stewart has not been charged and repeatedly has denied any wrongdoing. She has said that she and Bacanovic had an agreement by which he would sell her ImClone shares if the price dipped below $60.