Mexico City needs the controversial new airport currently under construction to avoid air traffic "bottlenecks" after 2021, Swiss investment bank UBS said on Monday.

Over the next three years, the capital's current airport will reach full capacity, UBS analyst Rogerio Araujo warned in a report.

"If the new airport is cancelled, we believe the airlines will be forced to seek alternatives by flying into other major cities," the report said.

Building Mexico City's New International Airport is a massive project that has already exceeded its budget and is still nowhere near completion.

Mexico's President-elect Andres Manuel Lopez Obrador, who will take office on Dec. 1, said he wants to hold a plebiscite in October to let Mexicans decide whether or not to continue the project, which has been marred by alleged corruption and technical problems.

The new airport is slated to begin operating in November 2020 following an investment of 13.3 billion U.S. dollars. Construction began in 2016.