Alliance leader Naomi Long has said an "anomaly" in the counting of votes for Derry and Strabane council in last month's election prompted the party to launch a legal challenge.
She said one of the stages of the count in Waterside area, where the party lost a seat to the SDLP, had not been "properly completed".
Alliance has lodged an election petition with the High Court in Belfast which could result in a recount.
