MEXICO CITY (Reuters) - Shares of billionaire Carlos Slim’s America Movil slumped on Thursday after regulators ratcheted up antitrust measures aimed at breaking Slim’s hold on Mexican telecoms, adding uncertainty to the outlook for his principal cash cow.

The America Movil logo is seen on the wall of the reception area in the company's corporate offices in Mexico City August 12, 2015. REUTERS/Henry Romero

America Movil was the cornerstone of Slim’s rise to top the global rich list for several years, but a 2013-14 telecoms reform imposed tougher antitrust rules on the company and helped open the door to new competitors.

Long-dominant America Movil AMXL.MX said late on Wednesday that the Federal Telecommunications Institute (IFT) had decided to step up antitrust measures against it, sending the company's shares down more than 5 percent at Thursday's market open.

The IFT said America Movil, the largest telecommunications company in the region, has to separate part of its network infrastructure into a new entity and that it will be subject to stricter checks on access to infrastructure.

This could open up its network further to competitors at lower prices, but it also raises questions about how the measures will take shape. America Movil now has 65 working days to present a proposal for its new unit.

"A big thing is the uncertainty it adds," said Carlos Legarreta, an analyst at broker GBM. "The implementation of this is surely going to be complicated," he said, adding that a similar measure against Britain's BT BT.L took a year.

America Movil’s shares ended down 4.54 percent at 12.63 pesos on Thursday.

Slim’s company has also long wanted to get a license to offer pay television in Mexico, a service it has been banned from since Telmex’s privatization in 1990. But after the new IFT measures, some analysts thought it would not happen soon.

Intercam analyst Alik Garcia said he did not see the company getting a license under the current president, whose term in office ends in November 2018, and that even once approved, it may not be enough to counter the company’s current woes.

“It’s a business that doesn’t have high margins ... it wouldn’t be a panacea,” Garcia said.

A spokeswoman for America Movil did not immediately return a request for comment.

The IFT also imposed new rules on the dominant broadcaster and longstanding Slim antagonist Televisa TLVACPO.MX, including increased accounting transparency, but Televisa shares rose as analysts thought it could benefit from America Movil's problems.

America Movil holds more than two-thirds of Mexico’s mobile subscriptions and dominates the country’s fixed line and internet services while Televisa has a more than 60 percent market share of pay TV.

In 2012, a study by the Organization for Economic Co-operation and Development concluded that Mexican consumers were overcharged for phone and internet services; America Movil disputed the findings.

Since the reforms, U.S. wireless operator AT&T Inc T.N has entered the market and phone service prices have fallen. But in broadcasting, changes have been more gradual.

IFT will strengthen oversight of Televisa’s infrastructure-sharing and require it to hand over detailed information on advertising prices, as well as provide separate accounts for each part of the business, the broadcaster said in a statement.

Televisa said it will consider legal or business actions it could take in response.

Slim’s company was stronger in its criticism of the regulator and vowed to challenge the decision.

“The modifications and additions to the measures ... confirm the lack of legal certainty and regulatory predictability in the sector,” America Movil said.