Fraudulent activity by web users in two countries in Europe has led Ryanair to restrict passengers in both markets, so they can only book up to seven days before a flight.
The low cost carrier says the restriction will affect passengers booking from the eastern European countries of Latvia and Lithuania after monitoring higher fraudulent activity than elsewhere on the network.
Eighteen destinations from Kaunas and six from Vilnius in Lithuania will be affected, as well as 15 from Latvia.
Some individual routes are also likely to be affected by the same policy, Ryanair says. It is hoped the move will reduce attempts by fraudsters to manipulate the systems as it will be easier to track and react to any suspicious activity.
The move coincides with an agreement announced this week between Ryanair and Cybersource to provide real-time monitoring of bookings.
VISA-owned Cybersource's technology (running up to 200 global validation tests when screening for fraud) will help accept, reject or mark individual cases for further investigation, the airline says.
Around 99% of the airline's bookings come via its own website, with the remainder through its call centre.