Which? Travel has levelled the criticism at the airline operator over a booking system that applies an exchange rate conversion at the point of payment.
Under the system, those booking flights from Europe to the UK on Ryanair's UK website are quoted fares in euros in line with standard industry practice.
However, that fare is converted to pounds by Ryanair at the point that payment for the fare takes place, with an exchange rate automatically applied.
That fare would be switched to pounds by Ryanair using an exchange rate of 93p to the euro, making the total fare £526.97.
If the customer had paid through Visa's exchange rate that day (88p to the euro), the trip would have cost £30 less overall at £496.97.
The Ryanair website automatically converts the price to pounds when it recognises the fare is being paid for by a British card.
Only by clicking on a hyperlink for "more information" on the Ryanair website can customers opt out of the dynamic price conversion and revert back to euros.
Which? Travel has reported what they described as "derisory" exchange rates to the Civil Aviation Authority (CAA).
Caroline Normand from Which? said: "This misleading pricing trick is one of the clearest examples of a rip-off we have seen, but Ryanair has been allowed to get away with it due to a lack of action from the Civil Aviation Authority.
"It must clamp down on this practice before thousands more holidaymakers are caught out this summer."
Ryanair said in a statement: [Our] currency conversion presentation is transparent and complies with applicable [consumer] EU and national laws."
The CAA said it would consider the findings submitted by Which?