Missouri Revised Statutes Title XXXV. Civil Procedure and Limitations § 507.090. Intervention--when as of right and when permissive--procedure

(1) When a statute confers an unconditional right to intervene; or

(2) When the representation of the applicant's interest by existing parties is or may be inadequate and the applicant is or may be bound by a judgment in the action; or

(3) When the applicant is so situated as to be adversely affected by a distribution or other disposition of property in the custody of the court or of an officer thereof.

(1) When a statute confers a conditional right to intervene; or

(2) When an applicant's claim or defense and the main action have a question of law or fact in common. In exercising its discretion the court shall consider whether the intervention will unduly delay or prejudice the adjudication of the rights of the original parties.

3. (1) A person desiring to intervene shall serve a motion to intervene upon all parties affected thereby. The motion shall state the grounds therefor, and shall be accompanied by a pleading setting forth the claim or defense for which intervention is sought. The same procedure shall be followed when a statute of this state gives a right to intervene.

(2) When the validity of a statute, regulation or constitutional provision of this state, or an ordinance or regulation of a governmental subdivision thereof affecting the public interest is drawn in question in any action to which the state or governmental subdivision or an officer, agency or employee thereof is not a party, the court may in its discretion notify the chief legal officer of the state or subdivision thereof affected.

(3) In all cases and proceedings wherein the validity of a statute, regulation or constitutional provision of this state affecting the public interest is drawn in question, and the state or an officer, agency or employee thereof is not a party, the state of Missouri may in the discretion of the court be permitted to intervene, upon proper application.

(4) In all cases and proceedings wherein the validity of an ordinance or regulation of any governmental subdivision of this state affecting the public interest is drawn in question, and the governmental subdivision, or an officer, agency or employee thereof is not a party, the governmental subdivision may in the discretion of the court be permitted to intervene, upon proper application.