Ryanair is halting its Belfast to Gatwick service from November until March.
It is one of 34 routes which to be suspended for the winter, after the company said it plans to "grow more slowly" this winter.
Ryanair's chief executive Michael O'Leary said: "Slower growth this winter, will create lots of spare aircraft and crews which will allow us to manage the exceptional volumes of annual leave we committed to delivering in the 9 months to Dec 2017."
St Agnes' Choral Society in Belfast are among those whose flight plans have been disrupted.
The choir's plans for a 60th anniversary trip to London are now in doubt.
Chairman Gareth McGreevy said news of the flight cancellations was a complete surprise.
The group had been due to travel to London on 1 November, but when one of the members went onto the Ryanair mobile app to check what time their return flight was home from London, a message said: "Flight cancelled. We sincerely apologise for the cancellation of your flight(s)."
O'Leary apologised to customers affected by last week's flight cancellations but said that today's announcement were "sensible schedule changes"
The company said it "plans to grow more slowly (4% instead of 9%) this winter, by flying fewer of our 400 aircraft between November and March 2018".
Those customers who have booked on the flights will be contacted today and offered "alternative flights or a full refund".
"If you have not received an email from Ryanair today then you are among the 99% of customers whose flights will be unaffected by these changes," it says.
The cancelling of an extra 18,000 flights will hit 400,000 customers.
Other UK flights include Newcastle to Faro and Gdansk.
