Ryanair boss Michael O'Leary has said it is a "damning indictment" of the Stormont Executive that it has done nothing to address Air Passenger Tax.
It was hampering NI airport growth by failing to scrap or cut the APD tax, he said.
He said politicians would have been better using the money to tackle APD.
It is currently £13 on each UK flight.
The executive has previously ruled out abolishing APD on all flights as too expensive, as it would have to repay the Treasury for implementing the tax-break.
Mr O'Leary also questioned the long-term future of City of Derry Airport and said it was likely that, over time, Ryanair's remaining flights there would move to Belfast International.
Last week, Ryanair announced it is to axe its service to London from City of Derry Airport. Summer flights to Faro, in Portugal, are also being dropped while the airline's service to Liverpool is being reduced to twice a week.
Mr O'Leary said the UK Brexit vote has curtailed further route expansion plans from Belfast International, citing the fall in the value of sterling and the expected slowdown of the British economy.
Good news for travellers?
However, the Westminster government said on Tuesday that it may intervene financially to maintain an air route from Derry to London.
In a statement, the UK's Department for Transport said the airport's operators, Derry City and Strabane Council, have been "invited to put forward proposals to keep the route open, with a value for money check to ensure the best outcome for taxpayers".
More than 125,000 people a year use the flight route, according to the department.
Minister for Aviation Lord Ahmad said: "Our airports are vital in supporting local economies, providing connections in the UK and opening up new opportunities.
"That is why the UK government has agreed in principle to support the air route between City of Derry Airport and London.
"In terms of process, it is important that Derry City and Strabane District Council take forward the tender process to secure this route."
Secretary of State for Northern Ireland James Brokenshire said the move was "good news for travellers in the north-west and the wider regional economy".
"Having talked to local business leaders I know the air link with London plays a vital role.
"I am pleased the UK government has been able to step in to help, as part of our commitment to build a Northern Ireland that works for everyone."
The Irish government abolished air tax in 2014.
In 2013, the Executive abolished APD on long-haul flights only - a move which costs it more than £2m a year.