Americans who refuse to pay their federal income tax because they sincerely believe the tax law is unconstitutional cannot be convicted of "willful" tax evasion, the Supreme Court ruled Tuesday.

The ruling does not shield them from paying taxes and penalties, however.The 6-2 decision set aside the conviction of John L. Cheek, an American Airlines pilot who filed no federal income tax returns for six years in the 1980s.

Writing for the court, Justice Byron R. White said someone's good-faith belief that federal taxes on their wages is unlawful - no matter how unreasonable that belief - are not guilty of a crime requiring willful action.

Even if a jury were to believe a defendant's claim that he sincerely believed he was not breaking any law, or that the tax law was unconstitutional, that would protect the defendant only from being convicted of willful tax evasion. It would not shield him from being convicted of not paying taxes, and having to pay back taxes with penalties.

Ignorance of the law or a mistake of law is no defense, White said.

But he added, "If Cheek asserted that he truly believed that the Internal Revenue Code did not purport to treat wages as income, and the jury believed him, the government would not have carried its burden to prove willfulness, however unreasonable a court might deem such a belief."

In other action, the court:

-Upheld the government's power to lift price controls on some natural gas. The justices, voting 8-0, reinstated federal regulations that allow gas producers to raise some prices charged to pipeline companies.

-Ruled unanimously that state prison inmates asked to testify in federal trials must be paid witness fees. The justices said the Denver-based 10th U.S. Circuit Court of Appeals was wrong when it ruled that federal law does not require such payments.

In the tax case, the jury that convicted Cheek in 1987 was told he should be found guilty of willful violations of tax laws unless his asserted misunderstanding of the laws was "objectively reasonable."

His conviction in a federal court in Chicago drew a sentence of a year and a day in prison and five years probation.

Tuesday's decision means federal prosecutors must retry Cheek, a commercial pilot since 1973 who stopped paying federal taxes in 1979. He became what lower courts called "a tax protester."