((a)) ** Jurisdiction** Except as provided in subsection (b) of this section, any person having a valid legal interest which is or may be adversely affected may commence a civil action for equitable relief on his own behalf in the United States District Court for the District of Columbia whenever such action constitutes a case or controversy—

((1)) against any person who is alleged to be in violation of any provision of this chapter or any regulation or condition of a license issued pursuant to this chapter; or

((2)) against the Administrator where there is alleged a failure of the Administrator to perform any act or duty under this chapter which is not discretionary.

((b)) ** Notice** No civil action may be commenced—

((1)) under subsection (a)(1) of this section—

((A)) prior to 60 days after the plaintiff has given notice of the violation to the Administrator and to any alleged violator; or

((B)) if the Administrator or the Attorney General has commenced and is diligently prosecuting a civil or criminal action with respect to such matters in a court of the United States, but in any such action any person may intervene as a matter of right; or

((2)) under subsection (a)(2) of this section prior to 60 days after the plaintiff has given notice of such action to the Administrator.

((c)) ** Right of Administrator or Attorney General to intervene** In any action under this section, the Administrator or the Attorney General, if not a party, may intervene as a matter of right.

((d)) ** Award of costs** The court, in issuing any final order in any action brought pursuant to subsection (a) of this section, may award costs of litigation (including reasonable attorney and expert witness fees) to any party whenever the court determines that such an award is appropriate.

((e)) ** Other remedies not restricted** Nothing in this section shall restrict any right which any person or class of persons may have under any statute or common law to seek enforcement or to seek any other relief.