Ryanair Aug. 11 began implementing a policy under which thousands of customers could end up at the airport expecting to take flights for which their tickets are no longer valid.

Under a new policy announced last week, as of Aug. 11 Ryanair is rejecting the tickets of thousands of customers who bought them from screen-scraper fare-comparison Web sites that book the flight themselves, instead of directing the customer to Ryanair's Web site to make the booking directly with the airline.

Ryanair's action also could leave thousands of travelers without a legitimate ticket for a flight they thought they'd already booked, even weeks or months ago -- and many of them may not find out until they show up at the airport.

That's because Ryanair is notifying customers by sending a notice to the e-mail listed with the booking; for screen-scrapers making their own bookings usually list the e-mail address of the screen-scraper, not the customer, Ryanair spokesman Stephen McNamara said. In that case, it will be up to the screen-scraper to notify the customer, he said.

Ryanair also is sending refunds for the canceled tickets to the credit card used to book the flight, which often is the screen-scraping site's credit card, McNamara said. That means customers looking for their refunds will have to go to the screen-scraper company to get it.

"We expect that we won't be able to identify every booking [from a screen-scraper site], McNamara said. "But where we find bookings we are canceling them." He said Ryanair gets about 200,000 bookings a day, about 1,000 of which come from screen-scrapers, he said.

The new policy is causing some confusion about which screen-scraper bookings are still valid.

Skyscanner, for example, felt compelled to announce that its customers' Ryanair bookings remain valid because it gives its users a direct link to official airline Web sites to book their flights; McNamara confirmed Skyscanner as a screen-scraper site that is still okay to use, and said Ryanair does not have a problem with sites like that. A Kayak.com spokeswoman, however, said Aug. 11 that Kayak recently stopped scraping Ryanair's Web site at the airline's request, even though it also sends its customers to airline Web sites to make direct bookings.

Ryanair's latest action is part of an ongoing battle -- frequently taking place in court -- between Ryanair and the sites.

Screen-scrapers take customers' requests for a flight and scan airline Web sites for the best fares, providing customers with a quick and very broad comparison. But Ryanair complains that it has not authorized most screen-scrapers to check its site, which violates its copyright and its terms regarding commercial use, and that the third-party bookings have caused problems.