Ryanair has raked in £18billion from add-on fees over the past decade, as passengers face "eye watering" charges to book seats and stow bags.
Analysis by the Telegraph has found so-called "ancillary revenues" have ballooned at the no-frills airline, up from £910million in 2013 to £3.28billion this year.
The figures have emerged amid heightened scrutiny of Ryanair's low-cost business model after an elderly couple was recently charged £110 for checking in for the wrong flight.
Ruth Jaffe, a retired GP, and husband Peter lodged a formal complaint after they had to pay airport check-in fees for mistakenly downloading return tickets instead of the outgoing version.
Ryanair has long boosted its balance sheet by charging customers for "optional" services, such as speedy boarding, on-board food and drink or seats next to each other.
However, these add-on fees have shot up in recent years, with Ryanair's average ancillary revenue per booked passenger rising 70% since 2013, now hitting £19.47.
Regain ground
This increase has coincided with flight tickets also rising, as airlines look to regain ground post-pandemic and pass through increased fuel costs.
Rival budget airlines, such as easyJet and WizzAir, have also prioritised charging customers more for add-ons in recent years.
Over the past six years, easyJet's ancillary revenue per passenger has risen 78% from £11.38 to £20.22.
As a share of total revenue, add-ons now account for almost a third of the company's turnover - up from a fifth in 2017.
Despite last week's controversy, Ryanair defended its policy to charge the couple, saying: "We regret that these passengers ignored their e-mail reminder and failed to check in online."
The resolute response was perhaps unsurprising given previous comments made by Ryanair chief executive Michael O'Leary, who once said: "You're not getting a refund.. We don't want to hear your sob stories. What part of 'no refund' don't you understand?"
Splitting up families claim
Back in 2017, the budget airline was accused of deliberately splitting up families who refused to pay extra for their seats. Ryanair denied this was the case.
Rory Boland, editor of Which? Travel, said: "Like many budget carriers, over the years Ryanair has mastered the art of extracting as much money as possible from passengers, adding on eye-watering fees for everything from a decent sized bag to sitting next to your kids.
"Take the time to consider what added extras you think you'll want for your trip and do some quick sums to compare the options on offer - you may find that it's cheaper to travel with an airline that includes things like hold luggage and seat selection in its standard fare."
A Ryanair spokesman said: "Ryanair has the lowest fares of any airline in Europe, delivering unbeatable choice for our customers.
"Our average fare is 49 euros (£41.84), demonstrating the exceptional value that Ryanair delivers to our customers. Additional products such as priority boarding, insurance, etc are all optional and passengers can mix and match ancillary products."
Meanwhile, Holiday companies have urged the prime minister to give extra powers to the aviation watchdog to directly fine airlines for service failures.
In a joint letter along with consumer organisation Which?, they called for stronger enforcement action if airlines fail to uphold consumer rights around refunds and cancellations.
It claims thousands of passengers have been subjected to "unfair treatment".
But the industry body for airlines said aviation was already highly regulated.
Many holiday providers cancelled flights and package deals this summer due to issues such as air-traffic-control restrictions and wildfires on Rhodes and other Greek islands, leaving British tourists in limbo.
The BBC says that, if a service is disrupted, airlines are supposed to re-route passengers - even on a rival airline; or offer food, accommodation and in some cases refunds if a flight is cancelled.
The Department for Transport recently recommended that the Civil Aviation Authority be given stronger enforcement powers, including the power to fine airlines that fail in their duties.