Fitch revised Mexico's rating outlook to negative two days after leftist President-elect Andres Manuel Lopez Obrador said he would cancel a partly-built new Mexico City airport in which billions of dollars have already been invested.

There were also risks that Lopez Obrador, who takes office on Dec. 1, would undermine economic reforms, such as the outgoing government's opening of the oil and gas sector to private capital, and could take steps that hurt growth, Fitch said.

"The decision to cancel the construction of a new airport for Mexico City sends a negative signal to investors," it said. Fitch has a BBB+ rating on Mexico's sovereign debt.

Lopez Obrador said he would cancel construction of the partially-built $13 billion new airport after an informal referendum organized by his party backed his plan to ditch the hub for a cheaper alternative scheme.

Less than 1 percent of the electorate voted for the cancellation of the airport in the referendum, which was widely criticized for lacking controls to assure it was fair.

The peso extended Tuesday's decline to fall over 2 percent to the dollar after the Fitch announcement, but pared some of the losses to trade around 1.5 percent down by midday (1800 GMT). It is on track for a loss of almost 9 percent in October, which would be its worst month since the election of US President Donald Trump in November 2016.

The country's benchmark S&P/BMV IPC stock index is on track for a 11.7 percent decline in October, its worst since January 2009 in the depths of the global financial crisis.

"The airport announcement came at a really bad moment for the peso because it basically clouded big investments and flows in uncertainty, erasing all the good will of the peaceful election in Mexico and the successful renegotiation of NAFTA," said Alfonso Esparza, an analyst at online forex broker OANDA.

Mexico, the United States and Canada agreed on an update to the North American Free Trade Agreement at the end of September, removing a key factor of uncertainty over the Mexican economy.

The market declines early in the month in Mexico tracked losses in riskier assets around the world, due to concerns about global trade tensions. But Mexican markets suffered major losses on Monday following the decision to cancel the airport.