Updated from 1:38 p.m. EDT

Sirius XM ( SIRI) said it has adopted a stockholder rights plan in what appears to be an attempt to fend off any hostile takeover of the company.

Under the plan, if any person or group acquires 4.9% or more of the outstanding Sirius XM shares without the approval of the company's board of directors, a significant dilution in the voting and economic ownership of the person or group would occur, Sirius XM said late Wednesday.

"This rights plan protects the interests of all stockholders and preserves these substantial tax benefits for the Company," said CEO Mel Karmazin in a statement.

While the market seemed to take the announcement as a poison-pill plan, Karmazin was careful to point out the rights plan is intended "to enhance stockholder value" and that the company did not implement it "for defensive or antitakeover purposes."

In February, Liberty Media helped save Sirius XM from a potential bankruptcy filing with a $530 million loan infusion giving billionaire John Malone's company a 40% stake in the satellite radio provider. The shareholders rights plan exempts future acquisitions of common stock by Liberty Media.

Liberty Media has three tracking stocks: Liberty Interactive ( LINTA), Liberty Capital ( LCAPA) and Liberty Entertainment ( LMDIA).

Before the transaction took place, EchoStar ( SATS), which sells set-top boxes and was the former parent of Dish Network ( DISH), had also reportedly been trying to gain control of Sirius XM, going as far as to accumulate a large amount of Sirius XM's debt.

EchoStar is controlled by Charles Ergen, who had offered to restructure Sirius XM's debt and inject several hundred million dollars of capital into the company in return for control, according to several media reports.

Liberty's rescue of Sirius XM came in two phases, with the first half used to pay off a chunk of Sirius XM's maturing debt. The second stage of the Liberty investment called for another $150 million loan to XM Satellite Radio, to be used to repay a portion of debt due in May, with an agreement to buy $100 million in XM loans outstanding.

Sirius XM issued Liberty 12.5 million shares of new preferred stock that converts into 40% of the company's common stock.

Sirius said it plans to submit the shareholder rights plan for a stockholder vote by June 30, 2010. If stockholders do not approve the rights plan by that date, it will terminate. The rights plan will last until Aug. 1, 2011, unless ended by the board, Sirius XM said.

Shares of Sirius XM closed down 1.4% to 39 cents a share. The stock is now higher by more than 200% in 2009, but has lost more than 80% since the July 2008 merger between Sirius and XM Satellite Radio.