Any person furnishing labor or materials for a public work who has not been paid therefor may file at any time prior to the expiration of twenty (20) days following the completion of a contract for public work a stop notice with the public agency concerned and thereby cause the withholding of payment to the contractor for the public work the amount claimed by such person. Any public agency receiving such notice shall mail a copy of this statute requiring action within twenty (20) days to the party making such claim. Any such person having no direct contractual relationship with the contractor, other than a person who performed actual labor for wages, may file such a notice, but no payment shall be withheld from any such contractor after the expiration of said twenty (20) days, unless the party making a claim shall have served the public agency with proof that he has instituted a legal action within such time to effectuate collection. Upon the filing of a legal action, the public agency shall withhold the amount claimed in the action, and pay the balance to the contractor. Failure by the claimant to comply with the requirements set out in this act and to post statutory guarantees shall release the public agency from the obligations imposed by the act.