The CNMC, as the regulator is known, said that Apple may have imposed unequal commercial conditions on developers of mobile applications sold in its app marketplace.

The practices could be considered a very serious violation of competition law and could carry a fine worth as much as 10% of the company’s global revenues, the regulator said in a statement.

Apple denied imposing unequal commercial conditions on mobile application developers.

“Spanish developers of all sizes compete on a level playing field on the App Store,” it said in a statement.

“Apple will continue to work with the Spanish Competition Authority to understand and respond to their concerns,” the statement said.

The CNMC investigation follows two different ones opened against the tech giant by the European Commission: one on whether the company breached the bloc’s landmark Digital Markets Act which seeks to ensure a level playing field for smaller rivals and another on new fees imposed on app developers.

In March, Brussels fined Apple €1.84 billion for thwarting competition from music streaming rivals via restrictions on its App Store, the iPhone maker’s first-ever penalty for breaching EU rules.