Passengers whose flights are delayed should be compensated automatically, it has been suggested.
The consumer rights group Which? claims airlines would be holding back millions of pounds in unclaimed compensation, with thousands of passengers being refused payouts.
Customers are entitled to up to 600 euros (£520) when flying from the UK or with an EU airline to an EU airport if their plane lands more than four hours late.
Which? has obtained figures from the Civil Aviation Authority which show airlines are told to pay in up to 83 per cent 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."
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 do so nearly three-quarters of the time.
Emirates said it 'complies with all legal requirements and regulations as set by the relevant authorities'.