In every case where a special tax return is not filed, or the tax is not paid, at the time prescribed in § 46.103, the delinquency penalties specified in § 46.107 for failure to file a return or for failure to pay the amount shown as tax on the return will be asserted and collected unless a reasonable cause for delay in filing the return or payment of the tax is clearly established. A taxpayer who believes the circumstances that delayed such taxpayer's filing of the return or payment of the tax are reasonable, and who desires to have the penalties waived, must submit with the return a written statement under the penalty of perjury, affirmatively showing all of the circumstances alleged as reasonable causes for delay. If the appropriate TTB officer determines that the delinquency was due to a reasonable cause and not to willful neglect or gross negligence, the addition to the tax will be waived. If the taxpayer exercised ordinary business care and prudence and was nevertheless unable to file the return within the prescribed time, or if the taxpayer made a satisfactory showing that the taxpayer exercised ordinary business care and prudence in providing for payment of the tax liability and was nevertheless either unable to pay the tax or would have suffered an undue hardship if the taxpayer had paid on the due date, then the delay is due to reasonable cause. Mere ignorance of the law will not be considered a reasonable cause.

(26 U.S.C. 6651)