Citigroup Inc., JPMorgan Chase & Co., and the rest of the banking industry face a new drain on their capital.

Borrowers from Sprint Nextel Corp. to Porsche Automobil Holding SE to MGIC Investment Corp. are drawing on credit lines. JPMorgan analysts say it’s the start of a trend that may force banks to raise as much as $40 billion to keep an adequate cushion against potential losses.

Companies are scrambling for cash at one of the worst times for the financial services industry. The world’s biggest firms have taken $195 billion in writedowns and losses on securities tied to subprime mortgages, and the 10 biggest American banks have the lowest capital levels in at least 17 years, according to Credit Suisse Group. The tapping of credit lines may be enough to grind new lending to a halt, a senior portfolio strategist at London-based hedge fund Asteri Capital, David Goldman, said.

“The capital of the financial system has imploded,” Mr. Goldman, a former head of debt research at Bank of America Corp., said in an interview on Bloomberg Radio in New York last week. “They have commitments to make loans, which they are being called out on, and their capital is bleeding to death.”

Bear Stearns Cos. was forced to sell itself to JPMorgan for $240 million, 90% less than its value last week, after the firm was crippled by clients pulling money and lenders reining in credit.

Banks had more than $1.4 trillion in untapped loan commitments as of September, the most since data became available in 1989, according to the Shared National Credit survey by four American regulators including the Federal Reserve and Office of the Comptroller of the Currency.

New York-based Citigroup had $471 billion at year-end, more than any other American bank, according to regulatory filings. Charlotte, N.C.-based Bank of America disclosed $406 billion of undrawn loan agreements and New York-based JPMorgan had $251 billion. Merrill Lynch & Co. had $59.3 billion. The banks’ Tier 1 capital, which includes common stock, retained earnings, and perpetual stock, shows why any further drain may “severely” limit new lending, said Credit Suisse analysts led by Ira Jersey.

The median Tier 1 level at the 10 biggest American banks fell to 7.3% of risk-weighted assets at the end of 2007, from 8.7% a year earlier, according to the analysts. The ratio hasn’t been as low since the Zurich-based bank began tracking in 1990. The minimum for a “well-capitalized” rating from regulators is 6%. The assets are calculated by weighing each type relative to its chance of default.

To compensate for the declines, banks have raised at least $50 billion in new capital from investors such as the Government of Singapore Investment Corp. and Abu Dhabi Investment Authority. Representatives for Citigroup, JPMorgan, Bank of America, and Merrill Lynch declined to comment.