Ryanair is to cancel thousands of its own customers' bookings after they were made through internet travel agents whose activities it says are illegal.
The airline is targeting price comparison websites on which you can buy Ryanair flights without having to go directly to the Irish firm's site.
Ryanair says this is against its terms and conditions, and the technology used slows down its site for other users.
But travel agents said the move was "foolish" and "unreasonable".
Consumer groups said they were "stunned" by the move.
So-called "screenscraping" websites account for about 0.5% of Ryanair's bookings, equivalent to about a thousand a day.
These websites use a technology that allows their booking tools to marry up to those of Ryanair and other low cost airlines, and execute a sale.
The customer, however, never leaves the original price comparison site.
But Ryanair insists these sites' activities are illegal and passengers using them are being forced to pay more for fares and other services.
Ryanair has taken legal action against Italian company BravoFly to force it to stop screen scraping the airline's website, having earlier taken similar steps against German firm V-tours.
Ryanair defended its decision to cancel bookings made this way for trips from Monday onwards, saying it was "a quicker and more effective way of discouraging this unlawful activity".
"We hope that by getting rid of screen scrapers we will speed up passenger processing times on Ryanair.com as well as ensuring that Ryanair passengers are not paying unnecessary handling charges or higher fares," spokesman Howard Millar said.
Passengers whose bookings have been cancelled will be compensated.
However, if the price comparison websites fail to tell passengers that their flights have been cancelled, there is little to prevent those affected turning up at airports all over Europe without a seat on a plane.
This is because Ryanair often does not have the contact details for passengers who book their tickets using these sites and as a result is unable to let them know directly of any cancellation or changes to their flights.
This is another reason why Ryanair said it dislikes these types of bookings.
The Association of British Travel Agents (ABTA) said Ryanair was being "foolish" and "unreasonable" while consumer group Which? also criticised the airline's policy.
"At a time when many people are struggling to afford holidays overseas, airlines should be finding ways to make it easier for families to book and travel in the way that is convenient for them, not making it even harder," said Rochelle Turner, from Which? Holiday.