Research by consumer advocate Which? has revealed that Ryanair and Tui, the UK’s largest tour operator, have been adding a surcharge of £20 to phone bookings, while EasyJet adds £15 to the cost of the fare and British Airways adds £10. Jet2 is the only airline not charging extra for phone bookings.

Phone surcharges could be increasing flight costs by 200 percent. For example, in the case of Ryanair, which advertises one-way flights to several destinations including Grenoble, Aarhus, and Stockholm for £9.78, phone bookings would raise the price for round-trip tickets to £39.78, a 204 percent increase. Carriers claim the fees cover the cost of call centers.

Ryanair said that “less than 0.5 percent” of its passengers book over the phone or at the airport, where the charge is also applied. “It has never been easier to book a flight at Ryanair.com or on our Ryanair app. As per our website, a €20/£20 applies to reservations made at airports/call centers to offset the associated administration costs,” a spokesperson said.

The low-cost carrier had 129 million passengers last year, which easily translates into hundreds of thousands of phone bookings that have netted Ryanair millions of pounds in fees.

Easyjet also reported that 0.5 percent of its passengers book over the phone, but that those who book online are entitled to a £15 internet discount. “We always encourage passengers to book online and those who use our telephone service are clearly informed of the online discount at the beginning of the call,” a spokesperson said. The airline added that passengers who require special assistance are also entitled to phone discounts on a separate line.

“We charge a small fee for bookings made by our expert sales advisors who provide an additional level of support and advice. This contributes towards the cost of running this service,” a British Airways spokesperson said.

Last year, Abta, a travel association, reported that 16 percent of travelers book their flights over the phone, down from 21 percent in 2016. “The over 65s are the group most likely to use the phone to book a holiday, with over a quarter (27 percent) doing so,” a spokesperson for the association said.

RELATED: I'm Not Paying For That! 19 Simple DIY Ways To Save While Traveling

In the past, kiss and drop charges were applied at airports to discourage loitering. At Luton, for example, it used to cost £3 for 10 minutes and £1 per minute thereafter to drop off or pick up passengers near the terminal. Nowadays, airlines have other ways of earning income, such as charging for blankets. Last year, Aer Lingus announced its “saver” fares would not include blankets or earphones, which now cost €5 and €3 respectively.

In addition, changing a name on a Ryanair ticket costs €115 or £115, while at EasyJet, it’s £47 online, or £20 if changed 60 days before departing. At Wizz Air, it’s €45.