The respondent in this action appeals from a judgment terminating her parental rights pursuant to subsections (2) and (4) of General Statutes § 17-43a (a)1 with respect to her eight *261year old son. Her claims of error are that (1) the criteria set forth in subsections (1) and (4) of § 17-43a (a) are arbitrary and unreasonable and therefore violate her constitutionally guaranteed rights; (2) there was insufficient evidence to support the termination order; and (3) the intervention of the foster parents as parties denied her a fair hearing. Because our view of the intervention issue is dispositive of this appeal we need not address the remaining issues, including the constitutional claims, and we reserve those issues for another day. During the hearing before the referee the foster parents, over the mother’s objection, were permitted to intervene as parties. The question before us is whether such intervention was permissible.
Although foster parents have standing in any proceeding concerning the placement or revocation of commitment of a foster child; General Statutes § 46b-129 (i);2 this standing does not spill over into a proceeding involving termination of parental *262rights. Thus the only question before us is whether the trial court has discretionary authority to permit the intervention of foster parents in such proceedings pursuant to Practice Book § 1023 (l).3 Our categorical answer to that question is “No.”
“It is . . . essential, in considering a petition to terminate parental rights, to sever completely the issues of whether termination is statutorily warranted and whether a proposed adoption is desirable. Although petitions for termination are presumably seldom brought unless prospective adoptive parents are available, there still must be a two-step process to determine, first, the threshold question of whether cause for termination under § 17-43a has been proved. ‘The best interests of the child, as such, is not an ingredient of [grounds for termination] and is not involved in this threshold question.’ Matter of Corey L. v. Martin L., 45 N.Y.2d 383, 391, 380 N.E.2d 266 (1978) . . . .” In re Juvenile Appeal (Anonymous) v. Commissioner of Children & Youth Services, 177 Conn. 648, 673, 420 A.2d 875 (1979). “Only if a ground for termination exists may the suitability and circumstances of adoptive parents, in an appropriate proceeding, be considered.” In re Juvenile Appeal (Anonymous), 181 Conn. 638, 645, 436 A.2d 290 (1980). The intervention of foster parents as parties at the termination stage will permit them to shape the case in such a way as to introduce an impermissible ingredient into the termination proceedings. “Petitions for termination of parental rights are particularly vulnerable to the risk that judges or social workers will be tempted, con-*263scionsly or unconsciously, to compare unfavorably the material advantages of the child’s natural parents with those of prospective adoptive parents and therefore to reach a result based on such comparisons rather than on the statutory criteria.” In re Juvenile Appeal (Anonymous) v. Commissioner of Children & Youth Services, 177 Conn. 648, 672-73, 420 A.2d 875 (1979).4
There is error, the judgment is set aside and the case is remanded with direction to deny the motion of the foster parents to intervene and thereafter to proceed according to law.
In this opinion Speziale, C. J., Peters and Armentano, Js., concurred.
“[General Statutes] Sec. 17-43a. teemination of paeental EIGHTS OF CHILD COMMITTED TO COMMISSIONER. (a) In respect to any child committed to the commissioner of children and youth services in accordance with section 46b-129, either the commissioner, or the attorney who represented such child in the prior commitment proceeding, or an attorney appointed by the superior court on its own motion, or an attorney retained by such child after attaining the age of fourteen may petition the court for the termination of parental rights with reference to such child, including the right to petition the court for the revocation of the commitment of the child. The superior court upon hearing and notice, as provided in sections 45-61d and 45-61f, may grant such petition upon finding that over an extended period of time, which, except as hereinafter provided in this subsection, shall not be less than one year: (1) The parents have abandoned the child in the sense that they have failed to maintain a reasonable degree of interest, concern or responsibility as to the child’s welfare; or (2) the parents have failed to achieve any such degree of personal rehabilitation as would reasonably encourage the belief that at some future date they could assume a responsible position in their child’s life; or (3) the parents, by reason of continuing physical or mental deficiency have, and for such period of time as will be detrimental to the best interest of *261the child, will be unable to provide him with the care, guidance and control necessary to his physical, educational, moral and emotional well-being; or (4) there is no ongoing parent-child relationship, which means the relationship that ordinarily develops as a result of a parent having met on a day to day basis the physical, emotional, moral and educational needs of the child and to allow further time for the establishment or reestablishment of such parent-child relationship would be detrimental to the best interest of the child; or (5) that both parents, or the sole parent of such child have consented to termination of parental rights with respect to such child. The court may waive the requirement that one year expire prior to the termination of parental rights if it finds from the totality of the circumstances surrounding the child that such a waiver is neeessary to promote the best interest of the child.”
General Statutes Sec. 46b-129 (i) provides: “A foster parent shall have standing for the purposes of this section in superior court in matters concerning the placement or revocation of commitment of a foster child living with such parent. A foster parent shall receive notiee of any application to revoke commitment or any hearing on such application.”
Practice Book § 1023 (l) authorizes the court iu its discretion to permit participation by an “[e]quitable party at interest” in the interests of justice.
The fact that we have not addressed the merits of the two grounds for termination does not mean that we regard either ground as factually or legally established. With respect to the failure of rehabilitation ground we reserve such questions as whether a nexus must be shown between the nonrehabilitation and the underlying cause for commitment and whether both the ground for termination and the underlying cause must be established by the constitutional standard of clear and convincing evidence. See Santosky v. Kramer, 455 U.S. 745, 102 S. Ct. 1388, 71 L. Ed. 2d 599 (1982).