Almost one million air passengers could be owed up to £420 for flight delays in the past year, according to Which.
The consumer group found that 37 million passenger journeys to and from the UK were delayed by 15 minutes or more between June 2014 and May 2015.
About 900,000 people could be eligible for compensation but only around 38% of them ever claim.
Passengers delayed for more than three hours are entitled to up to £422.
Over 9,000 flights are delayed for three hours or more each year with an average of 97 passengers on each flight, Which? research found.
Only four out of ten Which’ members surveyed said they had claimed compensation after a delay.
Half of those delayed said they received no support or information about the delay from their airline.
The group’s campaigns director, Alex Neil, told the BBC: “We want people to assert their rights and hold their airline to account for those delays and compensation that they are owed.”
Delays caused by “extraordinary circumstances” are not eligible for compensation under EU regulations.