MEXICO CITY (Reuters) - Mexican billionaire Carlos Slim sparred on Monday with the country’s presidential front-runner over the Mexico City airport his companies are helping build, saying the leftist’s threats to halt the $13 billion project would be a setback for the economy.

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Andres Manuel Lopez Obrador, who leads opinion polls ahead of the July 1 election, has said he will scrap the project, designed by architect Norman Foster and Slim’s son-in-law, charging that corruption and high costs make it unviable.

Slim’s family controls one of Latin America’s largest companies, America Movil. Companies owned by his family are co-designing, co-building and co-financing the airport project.

In a two-hour news conference at his offices in Mexico City, Slim said suspending the airport construction would halt growth in Mexico’s economy.

“The rationale of the airport project is indisputable. I don’t see many alternatives,” he said, adding that he was not talking to any of the presidential candidates.

Lopez Obrador responded by reiterating that the project could be done more cheaply at an existing military airport. He accused Slim of speaking on behalf of President Enrique Pena Nieto to stop the advancement of his political party, Morena.

“He’s a businessman, so he’s defending his interests, but I’m going to be president of Mexico and I’ll defend the interests of the people.”

Slim said on Monday that two of his companies that had contracts, Spain’s FCC and Grupo Carso’s Cicsa, had strict codes of ethics.

“I can assure you that we have not participated in any act of corruption at any time,” he said.

Lopez Obrador has not been specific on how he believes the alleged corruption took place, and has dismissed concerns that halting the contracts could create uncertainty, telling investors not to fear him.

Slim proposed that the new airport zone, one of the poorest in the metropolitan area, should be developed into parks, shopping centers and apartments.

Confirming a Reuters report from April, Slim said his bank Grupo Financiero Inbursa, through its pension fund and other funds, acquired a 43 percent stake in an investment trust designed to part-finance the project.

Slim, who has handed management of his business banking-to-mining-to telecoms empire to his sons and sons-in-law, said his companies’ investments totaled 8 percent of the investment in the airport, and that none of his or his family’s personal money was in the project.