MEXICO CITY, June 4 (Reuters) - Mexico's peso currency turned positive late on Sunday after election authorities projected a narrow win in a gubernatorial vote in the country's biggest state for the ruling party of President Enrique Pena Nieto.

The peso firmed to trade at 18.63 pesos per U.S. dollar, up 0.3 percent, after losing ground earlier on Sunday when the election for governor in the State of Mexico appeared to be too close to call. (Reporting by David Alire Garcia)