MEXICO CITY--Mexican tycoon Carlos Slim's telecommunications company América Móvil was ordered by a new powerful regulator to "substantially" cut some of its tariffs, the latest evidence that new telecom rules will trim the company's profit margins in its top market, Mexico.

America Movil was forced to cut the rates it charges to complete calls from competing networks for the period from April through December, the company said in a statement to the Mexican Stock Exchange. América Móvil said that starting April 6, wireless unit Telcel can charge competing carriers 20 Mexican cents--just under 2 U.S. cents--a minute for incoming calls, down from the current 31 Mexican cents.

Competitors such as Spain's Telefonica SA, Grupo Iusacell and NII Holdings Inc. unit Nextel México will be able to charge more for completing calls on their networks.

Interconnection rates at fixed-line unit Telmex were set at about a 10th of the mobile rates.

The rates are meant to help smaller telecoms compete with América Móvil, which controls around 70% of Mexico's fixed and mobile phone lines, and to lower fees for consumers.

The regulator could order new rates for 2015, or renew the ones it has mandated for the remainder of this year. The move was widely expected, although América Móvil said the rates represented a sharp decrease.

"The referred tariffs in the resolution are comparatively lower than the ones currently in force for the same services in most countries," América Móvil said.

América Móvil was declared dominant in its business in early March by a new telecom regulator created last year. As such, the company faces more severe regulatory conditions than smaller rivals in its industry as the regulator aims to level the playing field.

América Móvil shares traded on the New York Stock Exchange have fallen more than 12% this year, having closed Monday at $19.88.

The telecom regulator's decision comes at a moment when the Mexican Congress had started discussions on a telecom bill presented by President Enrique Peña Nieto that establishes tough regulation for dominant players.

The bill established that domestic long-distance charges will be phased out over three years and that América Móvil, like giant broadcaster Grupo Televisa, will be required to share infrastructure with competitors.

The new interconnection rates are still above the zero termination rates for dominant companies called for in the proposed new telecommunications law.

América Móvil last week described the zero-tariff measure as confiscatory, saying it would reward lack of investment by its competitors and discourage investment and innovation in the industry.

Anthony Harrup contributed to this article.

Write to Juan Montes at juan.montes@wsj.com

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