The battle between Belfast's rival airports has reached new heights after budget airline Easyjet announced it is switching a core route from Belfast International to George Best City Airport.
Easyjet said it will switch the twice daily London Luton service it currently operates from Aldergrove to Belfast City from January.
If successful, the airline plans to up its frequency from Belfast City and consider additional routes.
The move is likely to be seen as a blow to Belfast International, which posted a 15% drop in passenger numbers in August from the same month last year compared to Belfast City's 8% increase.
In a statement, Belfast International Airport said: "This sort of activity has been going on for years with all airlines in this market. While such a move to split operations and increase costs seems bizarre to us, we respect the commercial decision Easyjet have made."
It added: "We will continue to seek out sustainable routes for business and tourism for the whole region and will not be swayed in this strategic objective. On that basis we will step up our discussions with all airlines serving the market, in order to broaden our route network, focusing initially upon re-activation of the Belfast International-London Luton route."
Paul Simmons, regional general manager for Easyjet UK, said the airline was committed to the Northern Ireland market but not complacent.
"That is why we want to see if our passengers flying on selected shorter routes would prefer to fly via Belfast City Airport.
"The only way to actually do this properly is to run a route from there.
"We will take our time in assessing the passenger feedback and then take a view on how best to configure our operations between the two airports in the longer term," he said.
The airline currently operates a base at Belfast International, with five aircraft flying over 3 million passengers to 19 destinations.
The new route will put Easyjet in direct competition from Belfast City to London airports with Ryanair, which flies to London Stansted, Flybe which goes to London Gatwick and BMI which flies to Heathrow.
George Best Belfast City Airport chief executive Brian Ambrose said he expected the route to be popular.
However, residents living in east Belfast who are already resisting plans to extend Belfast City's runway, attacked the Easyjet decision.
Fiona McKinley, of the pressure group Belfast City Airport Watch, said: "It is a further step towards the day when Belfast will have a huge international airport right on its doorstep, creating an intolerable problem for the tens of thousands of local residents who already suffer from aircraft noise."