Budget airline passengers are paying more than the price advertised because of extra costs, a Which? survey claims.
Checking in luggage can incur extra costs
Firms are charging extra fees for items such as luggage check-in adding as much as £28 to ticket prices, Which? found.
Ryanair said charges were discretionary and largely avoidable, while Easyjet said the consumer group misunderstood the business model of budget airlines.
Ryanair reported passenger numbers up 18% in December on a year ago, while rival Easyjet saw numbers climb 9.9%.
However, shares in Easyjet were trading 13% lower in late afternoon trade at 466p after its load factor - which denotes the number of seats sold as a proportion of total seats available - fell to 78.9% in December, down from 81.2% a year earlier.
Ryanair shares dropped 5% to 3.88 euros. Its latest update, released on Friday, had shown its load factor dropping to 79% in December from 81% the year before.
Budget airlines have been increasingly charging for services that in the past have been included in the price of a ticket, Which? said.
According to Office of Fair Trading (OFT) rules, airlines must include taxes and other compulsory charges in their headline prices.
"If these costs are genuinely optional then we would not generally object to them being displayed separately, provided they are clear to consumers," said the OFT.
But the watchdog said if any of these charges were not optional and airlines advertised flights as "free" (or used a similar statement) or implied that consumers did not have to pay any extra charges then "this is likely to be misleading".
Ryanair was seen to charge the highest extra costs, by adding a £4 fee for checking in at the desk instead of online, and £20 for checking a bag into the hold.
Priority boarding cost an extra £2 per flight - it was only free for passengers who checked in online and had no bags for the hold.
Ryanair said it wanted to encourage passengers to travel with hand luggage only and to check in online.
The survey said overall total extra costs, including credit card charges, could be as much as £32 in Ryanair's case, just under £30 for Easyjet and just under £25 for BMIbaby.
Monarch Airlines, which in November was the latest firm to start charging for checking in baggage by setting a £3.99 fee per item, was seen to charge as much as £23.50 on top, while Flybe's total extras totalled were around £23.
Which? said it had spoken to the Air Users Transport Council which said nothing stops airlines from adding extra costs but that it would look into the issue.
"We think Ryanair's charge to use its check-in desk is unfair. You can only avoid it if you don't check luggage into the hold," Which? said.
Analyst Howard Wheeldon of BGC partners said "it looks as if the EasyJet trick of attempting to add revenue charging for checked luggage may not have gone down that well".
He added that if EasyJet needed to raise revenue and it couldn't do so "in the conventional competitive way" then it would have to raise ticket prices.