Yet more uncertainty in the banking system

One of the most important barometers of the world's financial health could be sending false signals.

In a development that has implications for borrowers everywhere, from Russian oil producers to homeowners in Detroit, bankers and traders are expressing concerns that the London inter-bank offered rate, known as Libor, is becoming unreliable.

Libor plays a crucial role in the global financial system. Calculated every morning in London from information supplied by banks all over the world, it's a measure of the average interest rate at which banks make short-term loans to one another. Libor provides a key indicator of their health, rising when banks are in trouble. Its influence extends far beyond banking: The interest rates on trillions of dollars in corporate debt, home mortgages and financial contracts reset according to Libor.

In recent months, the financial crisis sparked by subprime-mortgage problems has jolted banks and sent Libor sharply upward. The growing suspicions about Libor's veracity suggest that banks' troubles could be worse than they're willing to admit.

The concern: Some banks don't want to report the high rates they're paying for short-term loans because they don't want to tip off the market that they're desperate for cash. The Libor system depends on banks to tell the truth about their borrowing rates. Fibbing by banks could mean that millions of borrowers around the world are paying artificially low rates on their loans. That's good for borrowers, but could be very bad for the banks and other financial institutions that lend to them.

These bank problems are proving costly to other kinds of borrowers around the world. One way to measure the rough cost is by comparing the three-month Libor rate with an interest rate that doesn't reflect worries about banks' financial health -- such as the yield on a three-month Treasury bill, which is backed by the U.S. government. The gap between the two stood at 1.58 percentage points Tuesday, and has averaged 1.39 percentage points since the crisis began in August. In the five years before the financial crisis started, it averaged only 0.28 percentage points.

Citigroup's Mr. Peng believes banks could be understating even those abnormally high Libor rates. He notes that the Federal Reserve recently auctioned off $50 billion in one-month loans to banks for an average annualized interest rate of 2.82% -- 0.1 percentage point higher than the comparable Libor rate. Because banks put up securities as collateral for the Fed loans, they should get them for a lower rate than Libor, which is riskier because it involves no collateral. By comparing Libor with that indicator and others -- such as the rate on three-month bank deposits known as the Eurodollar rate -- Mr. Peng estimates Libor may be understated by 0.2 to 0.3 percentage points.