MEXICO CITY — Mexico’s president-elect Andrés Manuel López Obrador said Monday that he would cancel construction of an expensive new airport in the capital after voters rejected it in an informal voting procedure involving a small fraction of the population.

The decision to scrap the airport, a $13 billion project that is one-third complete, met with swift protests from Mexico’s business community, which warned that the cancellation could threaten the country’s economic stability by sending a message that investments may not be guaranteed under the López Obrador administration.

It also raised questions among investors about how much Mr. López Obrador, a leftist, will be guided by pragmatism and how much will he give in to his populist instincts after he takes office on Dec. 1.

Citing the results of an unofficial vote organized by allies of his own party, in which fewer than two percent of Mexico’s eligible voters participated, Mr. López Obrador said that his government would instead build two new runways at an air base north of the city to absorb the rising demand that has saturated the existing airport.