Delta Airlines' summer Edinburgh-New York flights will be extended year-round from this autumn, The Scotsman has learned.
The move came a day after Scottish airports said the Scottish Government's scrapping of plans to cut air passenger duty (APD) would stunt growth.
Delta is also launching summer flights between the capital and Boston tomorrow.
It has already announced its summer Glasgow-New York flights, which also resume tomorrow, will continue a month later this year, to 27 October.
But the airline has no plans yet to make the route year-round too, filling a gap since United ended winter flights from Glasgow two years ago.
The Edinburgh-New York move will mean Delta's sole remaining rival on that route, United, will now face competition throughout the year.
American Airlines this year switched its Edinburgh service to Philadelphia, while Norwegian ended its flights to Stewart in New York state last month.
Shane Spyak, Delta's European sales vice-president, told The Scotsman: "The JFK service has been doing really, really well for us."
He said the extended service had not been influenced by the other airlines' changes, but claimed Delta was the most punctual and reliable on the route.
Passenger numbers on Delta's Scottish routes also increased by 9 per cent last year - despite APD.
Mr Spyak said: "APD is not a direct factor [in route selection].
"We do not judge whether we are going to fly there or not on the basis of if there is a tax."
However, he added: "We would absolutely welcome a reduction in APD, which would make air travel more accessible."
Scotland's main airports, airlines and business groups wrote to First Minister Nicola Sturgeon yesterday claiming it was difficult to see how the country could win new international air routes "in light of the sudden reversal in policy which will serve to further exacerbate our connectivity deficit" [compared to other similar-sized countries].
"Our airports are already reporting a loss of routes and the promised cut in ADT would have provided airlines with the confidence required to invest in Scotland."
Meanwhile, welcoming the Boston route, Malcolm Roughead, chief executive of VisitScotland, said: “The US market continues to perform strongly for Scotland and remains the single biggest inbound market.
"Despite Brexit uncertainties, US operators tell us that consumer confidence in Scotland is high."
Mr Spyak said Delta had cut its carbon dioxide emissions by 1 per cent a year over the last decade, and it would replace nearly one third of its fleet with newer and more efficient aircraft over the next four years.