Toshiba Corp. (TOSBF) lurched lower again Friday after Standard & Poor's warned of a potential 'selective default' downgrade for the Japanese conglomerate if it seeks debt financing to assist in its global restructuring.

Toshiba shares fell more than 9% in Tokyo trading to end the session at ¥184 each, extending a decline that began in mid-December -- when the company revealed it would have to book a bigger-than-expected writedown on its U.S. nuclear operations -- to more than 60%. Toshiba said earlier this week that the goodwill impairment would amount to $6.3 billion.

"Given Toshiba's already very fragile financial standing, whether the company can receive continuous financial support from its creditor banks, including liquidity support, is a key factor in our credit analysis," S&P said. "Even in the event banks continue to provide financial support for the company, if it includes any form of debt restructuring we define as selective default, we will lower the ratings by multiple notches."

The company also said Friday that it would add a further 3% to its exposure to Westinghouse Electric Co LLC, its U.S. nuclear unit, after Japan's IHI Corp. exercised an option to sell the stake for $157 million.

Toshiba is hoping to sell a 20% stake its memory chip unit in order to shore up its balance sheet and has given itself a March 31 deadline to find a buyer. A sale of the entire division, which CEO Satoshi Tsunakawa said Tuesday was possible, could fetch the company more than $14 billion, analysts have said.

Toshiba has said it will likely post a net loss of around ¥500 billion ($4.4 billion) following a mammoth ¥700 billion write down to the value of Westinghouse and earlier this week its chairman, Shigenori Shiga, said he would step down as a result of the ongoing struggle to account for its overseas expansion.

Westinghouse, which Toshiba bought for $5.4 billion in 2006, overpaid to acquire a contractor that was working on several nuclear plants for it and now faces having to write down its investment and costs relating to projects that are badly behind schedule and over budget.

It is the Westinghouse acquisition of CB&I Stone & Webster assets that have caused it to fall behind schedule in publishing its financial statements.

"Internal reports were made to Toshiba's audit committee that suggested that internal controls related to the purchase price allocation process for Westinghouse's acquisition of CB&I Stone & Webster were inadequate," Toshiba said in a statement on Tuesday.

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