A U.S. Circuit Court of Appeals panel in Chicago has spiked one of the most powerful weapons national bank regulators have been using against directors of troubled banks: assessments of personal liability.

In a decision concerning First National Bank of Mt. Auburn, Ill., the nine-judge panel said the U.S. comptroller of the currency, who regulates nationally chartered banks, exceeded his powers in imposing $1.8 million in fines against five present and former directors of the bank without first filing suit against the directors in U.S. District Court.

A spokeswoman for the comptroller said the office had not reviewed the ruling, issued late Wednesday, but might appeal it to the Supreme Court. "Obviously, we don't agree with it," she said.

The comptroller had charged that, between 1979 and 1982, the directors, one of whom is now dead, repeatedly permitted the bank to make loans that exceeded its legal lending limit. At that time, banks could not lend any individual borrower an amount more than 10 percent of the bank's capital.

After several examinations and warnings, the comptroller administratively imposed $1.08 million in fines against four present and former directors and a separate $744,053 fine against William G. Butcher, president.

In overturning the fines, the appeals court, with one dissent, said the comptroller's office interpreted federal powers to correct problems at national banks too liberally. Assessing personal liability against directors "would eviscerate clear congressional intent," the appeals panel said. It called the assessments "a self-proclaimed assumption of power."

The court said the comptroller must return to Congress to get this power.

The spokeswoman for the comptroller's office said assessing personal liability against national bank directors had been part of its arsenal of enforcement powers for some time. She said appeals courts around the country had disagreed on the validity of the power, indicating an appeal to the Supreme Court might be in order.

The comptroller's other two options are to sue the directors in district court, a time-consuming and costly process, or ask Congress for broader powers.

Butcher said yesterday he was pleased by the ruling. Evidence in the case indicated that Butcher, who joined the board in January 1982, was unaware of the extent of lending to the borrowers involved in the case and had not been told of earlier comptroller's warnings.

The other directors assessed were Sam W. Taylor and Berniece Larimore. The former directors were Orville Bottrell and the late Albert P. Mulberry.