Merrill Lynch is expected to suffer $15 billion in losses stemming from soured mortgage investments, almost double its original estimate, prompting the firm to raise additional capital from an outside investor.

Merrill, the nation’s largest brokerage firm, is expected to disclose the huge write-down when it reports earnings next week, according to people who have been briefed on its plans. The loss far exceeds the $12 billion hit many Wall Street analysts had forecast.

To shore up its deteriorating finances, Merrill is now in discussions with investors in the United States, Asia and the Middle East, including American private equity firms, to raise about $4 billion in the coming days, these people said.

The developments underscore the rising toll that the mortgage crisis is taking on many once-proud Wall Street banks. In recent months Merrill and several other firms have grabbed financial lifelines from wealthy foreign governments. Further investments by so-called sovereign wealth funds could prompt scrutiny by Congress.