A consumer watchdog has said that it is "staggered" by moves by Ryanair to cancel bookings made through price comparison websites.
The low-fares airline has announced that from Monday customers will find bookings made through third-party "screenscrapping" sites will not be honoured.
The company will only allow flights to be purchased through their Ryanair.com website.
It has already taken legal action against two price comparison websites, Bravofly Ltd and V-tours, to prevent them harvesting flight information from Ryanair.com.
Deputy chief executive Howard Millar said: "Ryanair is pursuing legal actions in a number of European countries to bring an end to this unlawful and anti-consumer activity."
He said customers were paying handling charges, higher fares and getting inaccurate information from third-party websites.
"Ryanair will introduce new procedures to cancel all passenger bookings made through screenscraping websites. We believe this is a quicker and more effective way of discouraging this unlawful activity".
Rochelle Turner of Which?Holiday said: "We are stunned that Ryanair is treating its customers in this way. 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, not making it even harder."
The company will refund the websites for the cancelled flights but it will be up to each individual site to inform and refund their customers.
Sean Tipton, of the Association of British Travel Agents. said: "This move by Ryanair can only penalise their own customers and damage their reputation as an airline and we would hope that they would seriously reconsider what they are doing.
The low-fares airline has announced that from Monday customers will find bookings made through third-party "screenscrapping" sites will not be honoured.
The company will only allow flights to be purchased through their Ryanair.com website.
It has already taken legal action against two price comparison websites, Bravofly Ltd and V-tours, to prevent them harvesting flight information from Ryanair.com.
Deputy chief executive Howard Millar said: "Ryanair is pursuing legal actions in a number of European countries to bring an end to this unlawful and anti-consumer activity."
He said customers were paying handling charges, higher fares and getting inaccurate information from third-party websites.
"Ryanair will introduce new procedures to cancel all passenger bookings made through screenscraping websites. We believe this is a quicker and more effective way of discouraging this unlawful activity".
Rochelle Turner of Which?Holiday said: "We are stunned that Ryanair is treating its customers in this way. 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, not making it even harder."
The company will refund the websites for the cancelled flights but it will be up to each individual site to inform and refund their customers.
Sean Tipton, of the Association of British Travel Agents. said: "This move by Ryanair can only penalise their own customers and damage their reputation as an airline and we would hope that they would seriously reconsider what they are doing.