Mexico City - America Movil said on Sunday it removed roaming charges on calls to and data in the United States for 40 million Mexican prepay clients as it gears up to prevent new rival AT&T Inc. stealing market share on its home turf.

The company, owned by the family of billionaire Carlos Slim, said calls made to the United States by prepaid customers on "Amigo Optimo" and "Optimo Plus" plans would now be charged local rates. The same would apply when using data in the United States.

Its remaining prepaid customers, around 12 million at the end of the second quarter, could also request the change, the company said.

America Movil is currently facing a regulatory crackdown in Mexico, where it has 70 percent of the mobile market. Since the law was passed last year, AT&T challenged Slim by buying two Mexican wireless operators, prompting operators in both countries to start offering "borderless" call and data plans.

America Movil said in July it would eliminate roaming charges from Mexico to the United States for those of its 10.5 million post-pay customers who opt to pay an extra 50 pesos ($3.16) per month.