Denis Cheryshev (left) was ineligible to play against Cadiz

Real Madrid could be expelled from the Copa del Rey for fielding an ineligible player.

Russia winger Denis Cheryshev started in the fourth-round tie away to Cadiz despite being suspended after picking up a third yellow card in the semi-finals of last season's competition. He scored Real's first goal.

Cheryshev was on loan at Villarreal when he was booked for a third time in the 3-1 defeat to Barcelona in March, with suspensions carried over into the following season's competition.

Cadiz have submitted a written complaint to the Spanish FA over the offence and the governing body have set Real Madrid a deadline to respond.

President of third tier Cadiz, Manuel Vizcaino, admitted the decision was made with a "heavy heart".

"This situation required a speedy decision because there was no time to speculate," he said. "The board has met and decided to report Real Madrid for fielding an ineligible player, out of respect for our fans and the club.

Rafa Benitez is 'unlikely to be blamed' for the blunder

"We decided to do this with a heavy heart, but it is our firm decision. We do this not without a sense of pain, because we are friendly with the club (Real)."

Speaking after the game, Real manager Rafa Benitez said: "Clearly, the club did not know anything," he said. "We weren't told by Villarreal or the RFEF. We knew nothing and [after finding out], we have substituted the player to show good faith."

And Real director Emilio Butragueno told Marca: "We knew nothing of the situation.

Cheryshev (No 21) was substituted at the start of the second half

"We had no notification from the RFEF or Villarreal. Cheryshev, who was the player of Villarreal, he had no knowledge. We will await developments."

There is a precedent for such a situation in Spanish football, as Sky Sports' Spanish football expert Guillem Balague told Sky Sports News HQ: "The same thing happened to Osasuna in September, for fielding an ineligible player. They were kicked out of the competition as well."

Barcelona defender Gerard Pique tweeted his apparent reaction to the news

Real can appeal any judgement from the league but it is unlikely they will succeed, according to Balague.

He said: "We actually have a match delegate who is responsible for the line-ups and the referees, and actually checking if there are any suspensions, so I'm sure he'll be blamed.

Watch highlights of Real Madrid v Getafe in La Liga on Sky Sports 2 HD on Saturday, 9.30pm