The revocation of the authority of a representative covered by a power of attorney or tax information authorization filed in an office of the Bureau of Alcohol, Tobacco, and Firearms shall in no case be effective prior to the giving of written notice to the proper official that the authority of such representative has been revoked. [34 FR 6432, Apr. 12, 1969, as amended at 45 FR 7259, Feb. 1, 1980]