Travellers faced with delays can claim compensation / afp/getty images

Which? found that hundreds of thousands of travellers were eligible to claim compensation from airlines for delayed flights

Travellers are missing out on millions of pounds worth of compensation when their flights are delayed, a consumer group has found.

An investigation by Which? discovered that, between June 2014 and May 2015, 37 million passenger journeys to or from the UK were delayed by 15 minutes or more.

About 900,000 people could be eligible to claim compensation for this, but Which? found that only around 38 per cent - or four in 10 people – actually do.

Passengers delayed by more than three hours are entitled to £422 in compensation. 

But the study showed that half of those delayed said they received no support or information about the delay from the airline.

Richard Lloyd, Which? executive director, said: “We are urging people to hold their airline to account and claim compensation they are rightly owed if they have a lengthy delay.”

Asked why people do not claim compensation when flights are delayed, travel expert Simon Calder told the BBC: “It’s partly because people don’t know.

“The airlines aren’t very good at telling you your rights and I think lots of people simply give up.”

The consumer group conducted the investigation by analysing data for 1.7m flights over the past year and its calculations are based on data from the Civil Aviation Authority.

As part of its research, Which? found that Gatwick was the worst airport in the UK for delays of three hours or more, with 2,134 flights affected over the 12 months to May.

Passengers were most likely to experience delays of more than three hours on short-haul flights with airlines Vueling, Monarch and Thomas Cook, which together accounted for more than 700 delays in the last year across 68,000 passenger journeys.

For long-haul flights, passengers are more likely to be delayed by three hours or more with Pakistan International Airlines, Air India or American Airlines, which accounted for more than 400 flights across 40,500 passenger journeys

The three largest airlines operating in the UK – Easyjet, BA and Ryanair, which operated nearly half of all flights for the period analysed - accounted for four in 10 delays of more than three hours.

Which? has a free tool to enable people to claim compensation for flight delays.  To view it click here.