Barcelona could be thrown out of the competition amid claims they fielded an ineligible player in the first leg aginst Levante.
Levante have said they will lodge a complaint to the Spanish federation because Barca defender Juan Brandariz should have been serving a suspension after receiving a yellow card while playing for Barcelona’s B team in the Segunda B division.
Barcelona say a change in the rules, made in November, meant the suspension does not apply to the Copa del Rey.
Real Madrid were expelled from the cup in 2015 for fielding Denis Cheryshev when he was an ineligible.
The Catalans overturned a 2-1 first-leg deficit to beat Levante 4-2 and reach the quarter-finals of the Copa del Rey.
Two fortuitous Ousmane Dembele goals in the first half put Barca – cup winners in the past four seasons – ahead on aggregate at the Nou Camp.
Lionel Messi tucked in a third, his seventh goal in five games.
Barcelona were always in control of the second leg against Levante despite facing the prospect of their earliest Copa del Rey exit in nine years.
Messi had a good chance saved before Barcelona took the lead when Erick Cabaco’s clearance hit Dembele’s shin and lobbed over Levante goalkeeper Aitor Fernandez.
A minute later Fernandez inadvertently turned a chip from Dembele into his own net when diving to make a save.
In the 54th minute, Dembele crossed to Messi from the right before the Argentine shot into the bottom corner.