San Francisco - Netflix Inc., the world's largest subscription video service, adopted a so-called poison pill to protect against a hostile takeover after billionaire investor Carl Icahn acquired an almost 10 percent stake in the company.

The stockholder rights plan, approved unanimously by Netflix's board on Nov. 2, would be triggered if an "activist shareholder" acquired 10 percent of the stock, or an institutional investor bought 20 percent, Jonathan Friedland, a company spokesman, said in an interview.

The move is designed to make a hostile takeover too expensive and gives Netflix Chief Executive Officer Reed Hastings a tool to thwart Icahn or other potential buyers. Icahn, 76, said on Oct. 31 he had acquired stock and options representing 5.54 million Netflix shares. He said the video service is an attractive takeover target for larger companies, including Amazon.com and Verizon Communications, that have entered the market Netflix pioneered.

The shareholders' rights plan is intended to protect Netflix and its stockholders from efforts to gain control that the board deems not in the best interests of the company, Netflix said in a statement today. It's not meant to interfere with any merger approved by the board, Netflix said.

The measure will expire in three years unless Los Gatos, Calif.-based Netflix votes to extend it, according to Friedland.

The shares have whipsawed in the past two years, falling from a peak closing price of $298.73 in July 2011 to as low as $53.80 in September. The shares declined 12 percent on Oct. 24, the day after the company reported subscriber growth that disappointed investors also weighing its content and international expansion costs. The stock was up 11 percent this year before today.

Icahn spent $168.9 million to buy 1.25 million Netflix shares and 4.29 million options, according to his filing. The options expire in September 2014. Icahn didn't immediately return a call seeking comment on the poison pill.

The investor last year ended a battle for control of Lions Gate Entertainment Corp. after failing to win board seats, and bid unsuccessfully for software maker Mentor Graphics Corp. He said he invested in Netflix because the company is undervalued, based on its market position and prospects.