Cadiz president Manuel Vizcaino has confirmed that the Segunda Division B club will report Real Madrid to the Royal Spanish Football Federation (RFEF) over their selection of Denis Cheryshev in Wednesday's Copa del Rey tie.
Rafael Benitez's side won the first leg of the last-32 tie 3-1, with Cheryshev opening the scoring, but reports broke during the match that the Russian midfielder should have been serving a one-match suspension.
He collected three bookings while on loan at Villarreal during last season's competition, including one in the second leg of a semi-final defeat against Barcelona.
That triggered a one-match suspension that is yet to be served, although, speaking after the match, Benitez said "the club had no idea that Cheryshev was ineligible to play".
Cheryshev was replaced by Mateo Kovacic within a minute of the second half starting in what Benitez described as a move to "show good faith".
Vizcaino told a media conference after the match that he found the situation regrettable, but that Cadiz would pursue their case with the authorities.
"The board meeting has decided to make complaint about the illegal player Cheryshev, with pain in our heart," Vizcaino said.
Vizcaino went on to explain that he informed his Madrid counterparts of the apparent indiscretion, much to their surprise.
He added: "We will report Real Madrid. No one at Real Madrid called me. People of Real Madrid heard it from me. They were surprised.
"It's a hard decision because Madrid is the best club, but we have acted at the end of the match."