Family Dollar Stores Inc., facing a threat from billionaire activist Carl Icahn, adopted a shareholder-rights plan that will limit investors from acquiring more than 10 percent of the company.

All but one of Family Dollar's directors voted for the plan, which will last one year, according to a statement Monday. Edward Garden, who was added to the board as part of an agreement with Trian Fund Management LP in 2011, voted against it.

Family Dollar is adding the so-called poison pill after Icahn disclosed he'd amassed a 9.4 percent stake and would seek talks with management — a move that sent the company's shares up 13 percent. Icahn bought the shares because he plans to push for a merger with Dollar General Corp., according to a person with knowledge of the investor's thinking. He estimates that such a deal could generate savings of $500 million to $600 million, the person said.

"Carl Icahn is not someone who's taken lightly," Anthony Chukumba, a New York-based analyst at BB&T Corp. who has a hold rating on the stock, said last week. "He has a track record."

Family Dollar shares climbed to $68.62 Monday, their biggest gain in more than three years. The stock had slid 6.8 percent this year.

Family Dollar, a chain of budget stores based in North Carolina, has been struggling to compete with rival discounters, drugstores and big-box retailers such as Target Corp. and Wal-Mart Stores Inc.

Icahn

To combat slumping sales, the company embarked on a review of its business this year. As part of its turnaround plan, Family Dollar is closing about 370 underperforming stores and opening fewer new ones. It's also lowering prices in a bid to entice shoppers.

Trian, a firm run by billionaire Nelson Peltz, made a takeover offer in 2011, hoping to attract other suitors. Family Dollar CEO Howard Levine was reluctant to sell the company his father founded, and no other bidders emerged. The investor ultimately withdrew its offer as part of an agreement that added Garden to the board.