Letters have been sent to foreign oil companies, advising them of the opposition in Mexico to the energy industry reforms proposed by the government in August, leftist leader Andres Manuel Lopez Obrador said.

ExxonMobil Corp. chairman and CEO Rex Tillerson was told in a letter that the Mexican Constitution prohibits the participation of private companies in the oil industry, the former presidential candidate said.

Similar letters were sent to a dozen companies, including Chevron Corp., Shell, Total and Spain's Repsol, Lopez Obrador said in a press conference on Tuesday.

President Enrique Peña Nieto presented a bill to Congress that would change Article 27 and Article 28 of the Mexican Constitution, opening the way for the private sector to invest in oil and gas exploration and production projects via profit-sharing agreements with the government.

State-owned oil giant Petroleos Mexicanos, or Pemex, has a monopoly on the production of oil and gas, as well as on the production of petroleum derivatives and the distribution of gasoline.

Pemex, however, needs a large amount of investment and has seen its production drop.

Lopez Obrador said he warned Tillerson in his letter to "take into account the consequences of establishing business relations with a government that wants to violate the basic and spiritual principles" of the Mexican Constitution.

The ExxonMobil chief was told that signing a contract or investing in Mexico's oil industry "would be like buying goods without a receipt" and "tantamount to piracy," Lopez Obrador said.

Peña Nieto is pushing to reform Mexico's tax system and energy industry, with his proposals drawing the opposition of the left.

Lopez Obrador said the letters to foreign oil companies were sent as part of a campaign to stop the energy industry overhaul.

Peña Nieto, of the Institutional Revolutionary Party, or PRI, won the 2012 presidential election with 38.21 percent of the vote, while Lopez Obrador took second place with 31.59 percent. EFE