Ryanair has warned customers using its newly launched online booking system to expect delays "due to phenomenal demand".

The budget airline closed its Ryanair.com booking site at 10pm last Friday to replace its flight booking system from the Open Skies booking system to New Skies, both from specialist airline software supplier Navitaire. The upgrade was to cope with increased demand, and run more quickly "especially during peak booking periods" owing to "the increased processing power" of the new system.

But on its first day of operation, the airline warned it is working out issues with the new booking system.

"Due to phenomenal demand some customers are experiencing some delays during the booking process. We are working to resolve these issues as quickly as possible and ask that customers bear with us during this time," said the statement.

The airline launched its website at 7pm on Sunday, almost a day ahead of its original schedule of 11pm Monday, according to a Ryanair spokesperson.

"We expect to deal with some small bedding down issues over the next few days and we would ask passengers to please bear with us as we iron them out," said the spokesperson.

As part of the upgrade, Ryanair also made changes to its price-display to comply with a ruling by the Office of Fair Trading. Last year the OFT ordered 13 airlines, including Ryanair, to spell out their prices upfront online, instead of adding costs such as taxes and booking fees when customers go to pay for their tickets. Ryanair was the last of 13 airlines to change the pricing on its website to reflect the ruling.

The spokesperson said it had now made the changes, adding: "Fares including non-optional taxes, fees and charges are now displayed on the Flight Select page."

A spokesperson at the OFT said it was "too early" to tell if the changes met the requirements, but said that Ryanair had promised they would.

Ryanair said the new website would allow flight changes up to four hours before departure, online check-in up to five days before travel, and would have a specific section for checking and managing bookings.

The carrier upgraded its booking engine to cope with more web bookings, as the airline expects to double in size to reach 100 million passengers per year. Ryanair will also use the system to generate business intelligence reports on ticket sales and changes, which will help it to quickly launch promotions and process web payments more efficiently and cheaply.

Ryanair plans to make further changes to the site over the next few months, including adding functionality for customers to tailor their own flight, car and hotel packages, as well as adding 3D Secure payment verification, which adds security to online payments.