Ryanair has claimed that online travel giant Booking.com has bypassed the airline's website security system to continue alleged screen-scraping of the carrier's fares.
In a court case filed in the United States, Ryanair claims that its brand is being damaged by the company's actions and is seeking a jury trial in Delaware.
The airline has sued Booking.com, and its subsidiaries - Kayak.com, Priceline.com and Agoda.com.
Ryanair is already suing the Dutch arm of Booking.com in Dublin.
Screen-scraping involves a third party accessing an airline's website and often offering fares to its own customers via its own website. Screen-scraped fares are typically more expensive for customers than if they had bought tickets directly from the airline's website.
Ryanair has taken legal action against such screen-scrapers in the past. It says the third-party websites are not authorised to sell its tickets.
The practice has caused a fresh headache for the airline in recent months because of flights cancelled due to the coronavirus pandemic. Customers who booked their tickets indirectly via a screen-scraper website have experienced delays in securing refunds due to the way in which some of those sites are operated.
"Booking.com, Kayak.com, Priceline.com, and Agoda.com's refusal or failure to transfer the customers to the Ryanair website for the purpose of booking a flight with Ryanair interferes with Ryanair's commercial interests and damages Ryanair," the airline has alleged in its US court complaint.
It claimed that the defendants are depriving Ryanair "of the opportunity to maximise its revenues" from its own website.
The airline said it has spent a significant amount of money in an effort to block Booking.com and its subsidiaries from allegedly accessing its website.
"Ryanair has developed a program called Shield that blocks unauthorised third parties such as the defendants from scraping the Ryanair website and selling Ryanair inventory," it said.
Shield uses a machine-learning algorithm to determine if a user accessing the Ryanair site is a screen-scraper. If identified as such, the user is blocked.
"On information and belief, the defendants have taken measures to circumvent Shield so that they may continue their unauthorised access of the Ryanair website," the airline has alleged.
Ryanair has sued US-based Booking Holdings Inc as part of the legal action.
The airline's lawyers said Ryanair has instructed Bookings Holdings Inc (BHI) to cease its alleged screen-scraping practice.
However, lawyers for BHI last month wrote to Ryanair's legal representative, vehemently denying the airline's claims.
"BHI adamantly disputes the accusations in your letter, all of which are inaccurate," they said. "BHI is a holding company that does not operate a consumer-facing travel website and does not scrape the websites of other companies for data, either directly or indirectly."
"Each of the subsidiaries assumes control over its own respective brands, including all day-to-day operations," the lawyers said. "The subsidiaries' respective websites and mobile apps are each also owned and controlled entirely by the relevant subsidiary or one of BHI's other subsidiaries."
BHI's lawyers added: "Your attempt to conflate BHI with its subsidiaries and your suggestion that BHI can be held liable for any conduct that its subsidiaries allegedly took is misguided."
"Your dispute is with Booking.com BV such that your letter is a blatant attempt to get two 'bites at the apple' for claims that are already pending in Ireland."
Ryanair launched its legal action against Booking.com at the High Court in Dublin last November.
The airline is also suing an unrelated company, Skyscanner, in Dublin for alleged screen-scraping. Skyscanner operates an airfares comparison site.