The largest United States bank also reduced previously reported third-quarter profit because of worsening credit market problems, which it expects to reduce future cash flow.

'There's no way I think anyone can give you an assurance of how things are going to move,' Chief Financial Officer Gary Crittenden said on a conference call on Monday. 'We've taken what we think is a reasonable stab.'

Citigroup's struggles comes as the bank faces a leadership void following Chairman and Chief Executive Charles Prince's resignation on Sunday.

Mr Prince left after a four-year tenure during which the bank's shares fell 17 per cent amid criticism that Citigroup had grown unwieldy and lacked direction. Some investors want the bank, which has US$2.35 trillion of assets, to be broken up.

Former US Treasury Secretary Robert Rubin, who led the bank's executive committee, is now chairman. Sir Win Bischoff, head of the European business, became acting chief executive.

Citigroup shares fell US$2.52, or 6.7 per cent, to US$35.21 in afternoon trading, and hit a a 4-1/2-year low.

News of the expected US$8 billion to US$11 billion write-down, equal to US$5 billion to US$7 billion after taxes, helped drag down shares of such rivals as Bank of America, Goldman Sachs Group, Merrill Lynch and Morgan Stanley.

Investors worry that write-downs for subprime mortgages and other debt might not be isolated.

Citigroup on Oct 15 wrote off just US$1.56 billion for its subprime portfolio for the third quarter, part of an overall US$6.5 billion write-down. The new write-down reflects problems discovered since then.

Analysts said Merrill Lynch may add to its own announced US$8.4 billion write-down. That company ousted its chief executive, Mr Stanley O'Neal, last Tuesday.

'It shows the clumsiness of pricing mechanisms across Wall Street,' said Mr Michael Holland, a money manager and founder of Holland & Co in New York. He said it's difficult to value Citigroup 'until the dust settles'. -- REUTERS