The European Ombudsman has called for an investigation into allegations that some Spanish clubs are benefiting from "unfair tax advantages".
Emily O'Reilly has urged the European Commission to look into complaints made by investors in other European football clubs.
The EC competition office launched an inquiry in 2009 following allegations Spain is going against EU State Aid Rules by allowing unfair tax advantages for four Spanish clubs amounting to several billion Euros.
Furthermore, the complainant suggested that the lack of action from the Commission could be down to a conflict of interests, with the Commissioner reportedly supporting one of the clubs in question.
The issue is to feature on the agenda of the College of Commissioners on Wednesday and O'Reilly wants to know why the allegations should have been investigated long before now.
"The Commission has failed to act on this complaint for more than four years. Not only is this bad administration, but to the European public it can look like a conflict of interest given the Commissioner's strong links to one of the football clubs in question," she said in the statement.
"In my inquiry, I have not looked into the merits of the allegations concerning the breach of State Aid rules. I trust, however, that the Commission will decide to open an investigation tomorrow in order to investigate the facts and dispel any suspicions."