Ryanair is breaching Office of Fair Trading rules that all fares include taxes and charges when offered for sale.
The budget carrier blamed problems with its “system performance” and its website carries an apology. Ryanair said it hopes to resolve the problem shortly.
However, rivals can point to a history of non-compliance with the OFT ruling that requires airlines and other travel firms to include all non-optional costs in headline fares.
The ruling came into force in May 2007. Ryanair won a dispensation until the end of January 2008 on the grounds that its booking system required overhauling. However, its website did not comply until the last week of February. It is understood the system has failed to show the charges since the end of June.
Other major carriers have complied in the meantime and travel association ABTA has fined a handful of members up to £5,000 for failing to conform with the regulation in any part of their business.
Ryanair blamed the high use of its site for the problem. An airline spokesman said: “This is a non-issue. It is just a system error.”
An OFT spokeswoman said: “We have had a few calls about this and we know there are some issues. The site does carry an apology and we are working with them. We do not want to jump the gun.”
She added: “We are not making excuses. We will see what happens over the next few days.”
The European Commission has warned airlines the ruling will be extended across Europe from next May.