New York ? Martha Stewart, under federal investigation on suspicion of insider trading, resigned Thursday from the board of directors of the New York Stock Exchange.

“It has been a great honor and privilege to sit on the board, but the rigors of my own very busy and demanding corporate life require my full attention,” the queen of home decorating said in a letter to NYSE chairman and chief executive Dick Grasso.

The Justice Department is investigating Stewart’s sale of nearly 4,000 shares of ImClone Systems just before the stock plummeted last December on bad news about the company’s highly touted cancer drug. She has denied any wrongdoing.

A day earlier, an assistant to Stewart’s stockbroker pleaded guilty to accepting money and other valuables to keep quiet about the sale. The assistant has agreed to testify against others.

Stewart, who was a stockbroker before launching her own business, was one of 27 NYSE board members. Her term was to expire next year.

The news further hurt Martha Stewart Living Omnimedia. In trading Thursday, the stock was down 59 cents to $6.21 on the NYSE. The stock has fallen more than 60 percent since Stewart’s alleged connection to the ImClone scandal was first reported in June.

A spokeswoman for Stewart had no immediate comment.

Grasso said Stewart’s decision was voluntary.

New York Stock Exchange Chairman Dick Grasso responds to a question during a news conference at the exchange. Grasso announced Thursday that Martha Stewart had resigned from the exchange's board of directors.

“We are saddened to lose Martha Stewart, who has built a brand and a company admired around the world,” he said.

“Our board will miss Ms. Stewart’s counsel and insight.”

Stewart dumped her ImClone stock just before the news broke that the Food and Drug Administration would not review the company’s application for the drug Erbitux.

Stewart has said she had a standing order to sell the stock if it fell below $60. Her stockbroker’s assistant, Douglas Faneuil, initially gave investigators the same account but later said there had been no such order.

Former ImClone chief executive Sam Waksal, a friend of Stewart’s, has pleaded innocent to telling friends and relatives to dump millions in ImClone stock before the bad news about Erbitux became public.