(Reuters) – Mexico’s main stock index fell by more than 4% on Thursday, dragged down by a sharp sell-off in shares for the country’s main airport operators following government changes to the terms of their tariff base regulation.

Stock in airport operator OMA nosedived by more than 40% in early morning trading, while those of peers GAP and Asur tumbled by as much as 33% and 28% respectively after they on Wednesday evening flagged the changes by the civil aviation regulator.

Trading in the companies’ shares was briefly suspended, and the turmoil at one point sent Mexico’s S&P/BMV IPC stock index down by over 4%, before it pared some of the losses.

The airport operators did not go into detail about the changes. They said the modifications were being applied with immediate effect and that they were evaluating their impact.

Mexican brokerage Vector said in a note that the changes looked to be negative for the sector.

“The possible reduction in airport use fees (TUA), as well as non-aeronautical income, could have a negative impact on the profitability and generation of free cash flow at the airport groups,” Vector analyst Marco Antonio Montanez wrote.

“We recommend short-term caution in the face of abrupt movements that could be seen in companies’ prices.”