More than 900,000 people could be entitled to compensation for delayed flights, as new figures showed that about 43 million passengers suffered delays in the UK last year.
Nearly a quarter of the 1.9 million flights in and out of the UK were delayed by 15 minutes or more between April 2015 and March 2016, according to data from consumer group Which?.
Around 10,000 flights were at least three hours behind schedule, the research shows.
Passengers subjected to delays could be entitled to compensation, if the disruption was not caused by "extraordinary circumstances" such as weather conditions or airport strikes.
Compensation is dependent on the length of delay and the distance the flight is travelling, but can include meals, refreshments, phone calls and overnight accommodation.
Long haul travellers could be entitled to £250 if their plane landed between three or four hours late, or £510 if their flight was at least four hours behind schedule.
Passengers flying short haul are also potentially eligible to claim up to £210 if they were delayed by more than three hours.
There were 183,000 long haul passengers and 729,000 short haul travellers who were delayed by at least three hours in 2015/16, according to Which?.
Alex Neill, director of policy and campaigns at Which? said that many people were no claiming the compensation due to them.
"We know that tens of thousands of passengers on late running flights aren't claiming the compensation they're due and so we encourage people to claim what they're rightly entitled to," he said.