From August 11 the airline will cancel bookings made through websites which use screen-scraping, which entails lifting details from one website to produce a mirror of the original page on another.
Screen-craping websites allow bookings to be made with Ryanair without ever taking the customer to the airline's official website.
Ryanair claims that these sites' activities are illegal and that passengers using them are being misled into paying additional service charges and handling fees.
It has already taken legal action against Italian company BravoFly, which has now discontinued the practice, and has secured an injunction against German-based V-tours.
Ryanair has also highlighted Opodo.com, Atrapolo.com and OTBeach.com as websites that it plans to take action against.
The airline said that when passengers book through screen-scrapers it puts Ryanair's server under pressure and slows the website down, affecting the experience of other customers.
Passengers whose bookings have been cancelled will be compensated but Ryanair often does not have the contact details for people who use price comparison sites to let them know not to turn up at the airport.
The Association of British Travel Agents said that Ryanair was being "foolish" and "unreasonable".