Ryanair customers who have been forced to pay big sums for printing off boarding passes or changing details on travel documents could see the charges overturned in court.
Online firm Casehub is launching a legal action against the airline and signing up passengers who have been hit by the fees.
In recent years customers have been stung by charges of up to €70 (£55) to replace a boarding card or to check in, and €160 (£126) to amend documents.
In an attempt to improve its image, Ryanair has reduced some of these costs.
But Casehub says people who have already paid such charges should be able to reclaim them.
Its legal experts argue that the fees, which can triple the cost of a flight, are illegal under EU rules that bar companies from hitting customers with unfair contracts.
It says Ryanair has raked in £328 million from these charges, including interest growth on the cash, since 2009.
There is no upfront fee for signing up to the case, known as a class action, which will be heard in the Irish courts.
However, Casehub will deduct 40 per cent of any cash won in the courts as a fee and to cover legal costs. And it could take two years or more for the case to receive a verdict.
Sign up to the case at casehub.com/ryanair.
A Ryanair spokesman declined to comment.