(a) Restriction. Employees may not serve, other than on behalf of the United States, as an expert witness, with or without compensation, in any proceeding before a court or agency of the United States in which the United States is a party or has a direct and substantial interest, unless the employee's participation is authorized by the agency under paragraph (c) of this section. Except as provided in paragraph (b) of this section, the restriction in this paragraph (a) applies to special Government employees only if they have participated as an employee or special Government employee in the particular proceeding or in the particular matter that is the subject of the proceeding.

(b) Additional restriction applicable to certain special Government employees. (1) In addition to the restriction described in paragraph (a) of this section, special Government employees described in paragraph (b)(2) of this section may not serve, other than on behalf of the United States, as an expert witness, with or without compensation, in any proceeding before a court or agency of the United States in which their employing agency is a party or has a direct and substantial interest, unless the employee's participation is authorized by the agency under paragraph (c) of this section.

(2) The restriction in paragraph (b)(1) of this section applies to special Government employees who:

(i) Are appointed by the President;

(ii) Serve on a commission established by statute; or

(iii) Have served or are expected to serve for more than 60 days in a period of 365 consecutive days.

(c) Authorization to serve as an expert witness. Provided that the employee's testimony will not violate any of the principles or standards set forth in this part, authorization to provide expert witness service otherwise prohibited by paragraphs (a) and (b) of this section may be given by the designated agency ethics official of the agency in which the employee serves when:

(1) After consultation with the agency representing the Government in the proceeding or, if the Government is not a party, with the Department of Justice and the agency with the most direct and substantial interest in the matter, the designated agency ethics official determines that the employee's service as an expert witness is in the interest of the Government; or

(2) The designated agency ethics official determines that the subject matter of the testimony does not relate to the employee's official duties within the meaning of § 2635.807(a)(2)(i).

(d) Fact witness. Nothing in this section prohibits an employee from serving as a fact witness when subpoenaed by an appropriate authority.