Delayed airline passengers are potentially missing out on millions of pounds of compensation, according to an investigation by Which?.
The consumer group found that in the space of a year, between June 2014 and May 2015, 37 million passenger journeys to or from the UK were delayed by 15 minutes or more.
With more than 9,000 flights delayed for three hours or more, around 900,000 passengers are potentially eligible to receive compensation, it said, based on an average of 97 passengers being on a flight.
Which? also surveyed more than 7,000 of its members to find out more about their experience of flying with an airline from a UK airport.
It found only four in 10 (38%) people claimed compensation following a delay, which it said suggests consumers could be missing out on millions of pounds.
Half of those delayed said they received no support or information about the delay from the airline.
Passengers travelling within the EU who are delayed for more than three hours could be entitled to up to 400 euro (£282) in compensation, which rises up to 600 euro (£422) if the journey is over 2,170 miles.
The consumer group's calculations are based on Civil Aviation Authority data. Some delays for three hours or more would have been caused by "extraordinary circumstances" and would not be eligible for compensation under EU regulations.
While people can use a claims management company to claim for compensation, these firms can take up to one third of a consumer's payout.
Which? has free tools on its website to help people claim compensation if their flight is delayed.
It has a template letter to request compensation for a delayed flight at www.which.co.uk/consumer-rights/action/letter-to-request-compensation-for-a-delayed-flight/ and a letter to request compensation for a cancelled flight at www.which.co.uk/consumer-rights/action/letter-to-request-compensation-for-cancelled-flights-.