Spain has fined five budget airlines a total of €179m (£149m) for "abusive practices" including charging for hand luggage.
Ryanair has been given the largest fine of €108m (£90m), followed by EasyJet's penalty of €29m (£24m).
Vueling, Norwegian and Volotea were issued with sanctions by Spain's Consumer Rights Ministry on Friday.
The ministry said it plans to ban practices such as charging extra for carry-on hand luggage and reserving seats for children.
The fines are the biggest sanction issued by the ministry, and follow an investigation into the budget airline industry.
The ministry said it had upheld fines that were first announced in May after dismissing appeals lodged by the companies.
Vueling, the budget arm of British Airways owner IAG, has been fined €39m (£32m), while Norwegian Airlines and Volotea have been fined €1.6m (£1.3m) and €1.2m (£1m) respectively.
The fines were issued because the airlines were found to have provided misleading information and were not transparent with prices, "which hinders consumers' ability to compare offers" and make informed decisions, the ministry said.
Ryanair was accused of violating a range of consumer rights, including charging for larger carry-on luggage, seat selection, and asking for "a disproportionate amount" to print boarding passes at terminals.
Each fine was calculated based on the "illicit profit" obtained by each airline from these practices.
Ryanair boss Michael O'Leary said the fines were "illegal" and "baseless", adding that he will appeal the case and take it to the EU courts.
"Ryanair has for many years used bag fees and airport check-in fees to change passenger behaviour and we pass on these cost savings in the form of lower fares to consumers," he said.
Easyjet and Norwegian said they would also appeal the decision.
The Spanish airline industry watchdog, ALA, plans a further appeal and has called the ministry's decision "nonsense", arguing the fine infringes EU free market rules.
But Andrés Barragán, secretary general for consumer affairs and gambling at the ministry, defended the fines, saying the government's decision was based on Spanish and EU law.
"It is an abuse to charge €20 for just printing the boarding card in the airport, [it's] something no one wants," he told the BBC's World Business Report programme.
"This is a problem consumers are facing not only in Spain but in other EU countries."
Consumer rights association Facua, which has campaigned against the fees for six years, said the decision was "historic".
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