When there is little liquidity, lenders charge less creditworthy borrowers large premiums.

Chemical, like many other banks, benefited from a decline in the cost of the funds it borrowed relative to the interest it charged borrowers.

The lower rates have also widened the spread between the interest rates the banks pay depositors and what they charge borrowers.

Banks have enjoyed unusually high profits over the last few years, primarily because their cost of deposits has fallen faster than the interest they charge borrowers.

The prime rate is the benchmark that banks use to decide how much to charge borrowers, particularly consumers and small-business clients.

"So, now when they have cheaper funds, they are able to charge borrowers less," he said.

Low short-term rates widened the spread between the rates the banks paid depositors and the rates they charged borrowers.

As a result, the difference between what the banks charge borrowers and what they pay to savers is the highest it has been in years.

Japanese banks, Sumitomo Trust included, are also having difficulty raising the amount they charge borrowers.

Payday loan companies such as Wonga have attracted increasing criticism in recent months for the high interest rates they charge borrowers.