EU consumer protection officials have warned consumers that they are being defrauded by some online ticket sellers.

The EU consumer agency has said one in three airline ticket booking sites in 13 participating countries had breached consumer law and that regulators had to pursue them to enforce the rules.

The EU would has now threatened to intervene if the industry itself does not clean up the unscrupulous practices.

It has been revealed that low-cost carrier Ryanair, Austrian Airlines and Finland's Blue 1 had broken consumer rules.

Norwegian authorities found that Ryanair included the right to priority boarding as a pre-chosen option and charged a fee of 6.4 euros for it.

Similarly, Blue 1 included insurance in case of cancellation as a pre-chosen option for customers booking tickets online.

Norwegian authorities found that Austrian Airlines had a booking fee of 100 kroner per ticket that was not included in the advertised prices on the front page of the airline's website.