NEW YORK (TheStreet) -- Beleaguered bond insurer Ambac (ABK) , following carmaker Ford (F) - Get Report and satellite-radio broadcaster Sirius XM (SIRI) - Get Report, proposed a shareholder plan to make a takeover less attractive.

Under Ambac's shareholder-rights plan, when a person or group has obtained ownership of 4.9% or more of common stock, or an existing holder with greater than 4.9% acquires more shares representing an additional 1% of stock outstanding, a dilution in the economic interest and voting power would take effect. The proposal also would protect Ambac's net operating losses, NOLs, totaling $4.5 billion, which are used to offset taxes.

Private-equity firm

Blackstone Group

(BX) - Get Report

has been appointed by Ambac to complete a restructuring, Reuters reported yesterday. Ambac spokesman Peter Poillon said today that nothing specific has led up to today's announcement. It was a decision taken for the benefit of shareholders, he said. The spokesman declined to comment on the Reuters report.

Still, Ambac is concerned that its shareholders would lose rights if the insurer were taken over by a predatory corporate raider, the spokesman said. The plan outlined today will reduce any perceived benefits of a takeover and also protect against casual market trading.

Ambac's fourth-quarter results could be the most important of its life. An improvement may indicate that Ambac will survive. A deterioration may suggest any Blackstone involvement would be a preparation for bankruptcy. Poillon said Ambac's earnings report may be released in the first week of March.

Further details about Ambac's announcement will be released in an 8-K filing. There's speculation about whether Blackstone's involvement would be a positive or negative sign. Today's announcement, seen on the back of that report, has investors nervous. Short interest continues to grow in this speculative stock.

-- Reported by Gavin Magor in Jupiter, Fla.

Gavin Magor is the senior analyst responsible for assigning financial-strength ratings to insurance companies. He conducts industry analysis and supports consumer products. Magor has more than 22 years of international experience in operations and credit-risk management, commercial lending and analysis. His experience includes international assignments in Sweden, Mexico, Brazil and the U.S. He holds a master's degree in business administration from The Open University in the U.K.