The travel site claims to guarantee the lowest prices for flights, holidays, hotels and car rental worldwide, but it matches prices only from online travel retailers and travel agents, excluding Ryanair and bookings made directly through airline sites.
Ebookers pledges to offer consumers a refund if they find a like-for-like product at a lower price. The claim must be made within 48 hours of booking and at least 72 hours prior to the scheduled time of the departure. Airline websites are excluded from the guarantee.
A Ryanair spokesman said: 'Ebookers' flights 'price promise' is meaningless and misleading, and should be removed at once.' Ebookers was unavailable for comment.
Separately, last week Ryanair launched what it claimed was the world's first 'free seat sale', giving away 1m seats on routes across Europe.
The airline, which has come under fire in the past for omitting taxes and hidden fees from similar offers, said these seats were genuinely free to customers, who would pay no extra charges if they booked by 21 May.