Mexico City - Mexico's Supreme Court on Wednesday ruled billionaire Carlos Slim's telecommunications company America Movil should not be barred by law from charging competitors certain fees, prompting fears of a rollback of a sweeping antitrust reform.

The 2014 telecom industry reform, one of Mexican President Enrique Peña Nieto's signature accomplishments, prohibited America Movil from charging other carriers for calls made to customers on its network, even though those firms are allowed to bill America Movil for using their networks.

The reform was aimed at breaking up Slim's market dominance, which rivals said made it challenging for them to break into the sector. It was one of several changes, along with an opening of the state-run energy industry, that have increased competition in Mexico's strategic industries.

The Supreme Court ruling said the so-called interconnection rates should be set by telecom regulator IFT, siding with America Movil - Latin America's largest telecom firm by number of subscribers and the crown jewel of Slim's empire. Tariffs set by IFT will take effect from January 2018, the court said.