By Elinor Comlay MEXICO CITY, April 18 (Reuters) - Worries about a regulatory reform plan that seeks to curb America Movil's dominance in Mexico are more likely to weigh on the company's share price than the weaker quarterly profit it is expected to report on Thursday. Even if Latin America's biggest phone company beats expectations of a 25 percent drop in first-quarter profit, its share price, which has sunk almost 17 percent since the start of the year, will remain under pressure, analysts said. Lawmakers in Mexico's upper house are currently discussing a bill that seeks to dilute the dominance of billionaire Carlos Slim's America Movil in the mobile and fixed-line markets and encourage competition. "That's definitely a bigger deal than whatever the quarterly results are going to show," said Imari Love, analyst at Morningstar in Chicago.America Movil, controlled by Slim, the world's richest man, has spent more than $1.6 billion this year buying back stock to support its share price, but it has barely stemmed the slide. The value of Slim's stake in America Movil has lost more than $5 billion as the shares have tumbled 17 percent since the start of the year, while the Dow Jones Telecommunications Titans index is up 5.5 percent. Slim's flagship company plans to increase its buyback fund by 40 billion pesos ($3.27 billion) this year but the level of uncertainty over the content and implementation of the reform plan will limit the success of the company's buyback plan, according to analyst Christopher King at Stifel in Baltimore. "There's simply an issue with the overhang that's out there," said Christopher King, analyst at Stifel in Baltimore. It is not just worry over America Movil's future in Mexico, regulators in Colombia have also recently moved to curb the fees the company can charge rivals to connect to its network, King noted. America Movil is also facing increased competition in Brazil that is hurting the company's core profit margin as providers there fight for customers by offering subsidies for internet-enabled handsets. An additional drag on the company is its investments in European phone companies Telekom Austria and KPN , analysts said. America Movil last year spent 84.9 billion pesos accumulating shares in the companies. It holds almost 30 percent of Dutch company KPN and Telekom Austria. Shares in KPN and Telekom Austria are down 26 percent and 13 percent this year. "They spent a lot of money in Europe with these minority investments that I don't think they've done a good job of explaining," said King. "I think there's a general sense they would have been far better off using that cash to buy back stock as well."