Ryanair will from today begin quoting all of its airfares inclusive of taxes, the company said today.
The company and other 'low fares' airlines have in the past been criticised for advertising fares as low as one cent, when additional 'hidden' taxes and charges can add a significant sum to the total.
Ryanair said fare offers on its website will be quoted including taxes and promised that if passengers find a cheaper fare on the same route with any other airline, it will pay "double the difference" between the two fares.
However, Ryanair's commitment comes with numerous 'small print' qualifications.
The return fare quoted on its website will, it said, include all "mandatory taxes, fees and charges" but will exclude "credit/debit card fees, baggage fees, web check-in/priority boarding fees or any other optional fees".
The customer must also submit the claim within an hour of making the booking on Ryanair's website, or else it will be deemed late and will be "disregarded for the purposes of this promotion".
Claims will also only be accepted where both departure times of the competitor airline's flights are within one hour of the original booked Ryanair flight.
And Ryanair stipulates that anyone attempting to make the 'double the difference' claim must email a 'screen print' of the competitor's offered fair with their claim form.
It promises to verify the competitor fare "within one hour of receipt (only during office hours: 9am-5pm Monday to Friday)."
"If the 'competitor fare' cannot be verified on the internet then the claim will be invalid," Ryanair said in its conditions.
In addition, claims can only be made for flights where there is a "direct city pair" comparison, that is for airports that have the same city designation as defined by the International Air Transport Association (IATA). For example, Dublin's airport designation code, for example, is 'DUB'.
Ryanair will also cap successful claims by passengers seeking to 'double the difference' between the competing fares to a maximum of €100 (stg£70).
In relation to all claims, the airline's decision "will be final".
Aer Lingus was criticised by consumer lobby groups recently when it introduced an additional charge to passengers wishing to select their seat online when booking flights. The airline charges extra for aisle seats and seats at emergency exits which have additional legroom.
A spokeswoman for Ryanair said the 'all-inclusive' fare, to include taxes, is the "advertised" fare on Ryanair's website homepage.
Chosen randomly by ireland.comfollowing Ryanair's announcement, a flight from Dublin to Billund in Denmark on May 16th, returning on May 20th, was advertised on the website's homepage for €15 each way.
On proceeding through the booking procedure, the final quote for the return flight is €90.98, including additional charges of €22.74 on one leg and €16.95 on the other.
Separately, Aer Lingus said it was increasing its charges due to recent trends in oil prices.
The fuel surcharge on long haul flights to and from Ireland to New York, Boston, Chicago, Orlando, Washington and Dubai will rise to €40/US$46 and on flights to/from Ireland to Los Angeles and San Francisco to €50/US$67.
Aer Lingus commercial director Enda Corneille said: "The oil price continues to be extremely volatile and in recent weeks the cost of fuel has risen significantly.
"We said last May that we would keep our surcharge under review to reflect changes in the price of fuel, and today's decision to increase the charge is consistent with that strategy. We remain committed to reviewing levels of our surcharge in line with current market trends."
Ryanair called on Aer Lingus to remove its "unfair" fuel surcharge and said it would never impose a fuel charge - "not today, not tomorrow, not ever".