Airlines should give automatic compensation to passengers when their flights are delayed, it has been suggested.
Consumer group Which? claims airlines could potentially be keeping millions of pounds in unclaimed compensation and said thousands of passengers were being refused payments.
Customers are entitled to up to 600 euros (£520) when flying from the UK or with an EU airline to an EU airport and their flight lands more than four hours' late.
Figures from the Civil Aviation Authority obtained by Which? show that airlines are forced to pay in up to 83% of cases, but carriers can refuse as they are not legally obliged to pay compensation.
Alex Neill, Which? managing director of home products and services, said: "Some airlines seem to be making it as difficult as possible for passengers to receive the money that they are rightly entitled to for flight delays.
"We want to see airlines introduce measures so that, where possible, passengers are compensated automatically for delays and cancellations."
The figures show that Norwegian Air was advised by the regulator to pay up in 83% of cases. Vueling and Ryanair were told to pay out in 79% and 77% of cases respectively.
Which? said the "worst offender" for not paying out when asked to do so was Emirates, which was advised to pay compensation in six out of 10 cases, but refused to pay out nearly three-quarters of the time.
A Norwegian spokesman said: "We understand that punctuality is vital for our passengers and we strive to operate all flights on schedule.
"We take customer care very seriously and we always maintain a consistent policy regarding delays and cancellations in accordance with EU261."
A Vueling spokesman said: "Due to the busy period of the summer season and operational disruptions experienced by Vueling in early July 2016, it took the company longer than usual to reply some of our customers´ claims."
Ryanair said the 504 complaints made during the period concerned came at a time where the company carried more than 83 million passengers, while Emirates said they "comply with all legal requirements and regulations as set by the relevant authorities".