Norwegian is trimming regional transatlantic services due to poor consumer demand.
Flights from Belfast to Boston and New York will be withdrawn from October 27.
Transatlantic flights from Edinburgh to the two US destinations will also end from March 30, 2019 together with two Spanish services. The airline blamed a failure of the Scottish government to lower air taxes as the reason.
A spokesman for the budget carrier said: “Following a comprehensive review of our services from Belfast in response to customer demand, we have decided to withdraw our routes from Belfast to the New York and Boston areas.
“Customers can still book flights between Belfast and the US on our website for travel up to October 26.
“New York and Boston-Providence remain sell served from Dublin, Shannon and Cork with up to 33 weekly departures, giving customers in Northern Ireland an alternative for booking daily affordable flights to the US.”
On Edinburgh, the Norwegian spokesman said: “Our affordable US flights were launched with the prospect of a reduction in air passenger taxes that was unfortunately postponed by the Scottish government and this has led us to fully withdraw our transatlantic services.
“We continually monitor and evaluate routes across our network, and as such we have also decided to withdraw routes from Edinburgh to Barcelona and Tenerife allowing us to focus on maintaining better-performing flights to Scandinavia.
“As a growing airline, we are engaging with our Edinburgh-based crew about opportunities to relocate into other markets where air passenger taxes do not present a barrier to our transatlantic growth. Norwegian will continue to offer Scottish consumers direct, affordable flights to Scandinavia.
“Our withdrawal of transatlantic services from Edinburgh is a case in point that a reduction in air passenger taxes can foster lower fares for passengers and improve Scotland’s global connectivity.”
A spokeswoman for Belfast international airport told the Belfast Telegraph that it was “extremely disappointed” to lose the flights and it was an “unsustainable” situation for Northern Ireland consumers having to travel to Dublin for their nearest transatlantic service.
“The basic fact remains that the flight schedule provided by the airline from Belfast [two flights a week] was either flexible nor attractive enough to appeal to the local travelling public.
“There is significant – and prospectively increasing – market demand between Northern Ireland and the USA/Canada, particularly focused upon New York and Toronto as gateways.”
She added: “ We will continue to work to ensure that partner airlines are encouraged to serve our market in an appropriate and sustainable manner.”