The research shows that 24% of the 1.9 million flights into or out of the country were delayed by 15 minutes or more, affecting 43 million passengers, between April 2015 and March 2016.
Of the 449,000 delayed flights, more than 10,000 were more than three hours late, leaving those affected potentially in line for compensation under the current EU Denied Boarding Regulation.
However, where flights are delayed due to factors such as extreme weather or airport strikes, they’re unlikely to get compensation.
Of those who do qualify, 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.
The analysis showed that 183,000 long-haul passenger journeys were delayed by at least three hours.
Passengers flying short-haul are also potentially eligible to claim up to £210 if they were delayed by more than three hours.
The research showed that 729,000 short-haul passenger journeys were affected by such a delay over the period we analysed.
It named Southend airport as the location where passengers were most likely to be delayed on short-haul flights, with 1.7% of journeys delayed by three hours or more.
Meanwhile, Gatwick and Manchester airports suffered the most delays to long-haul journeys, with 1.3% of journeys delayed beyond three hours.
Vueling with 1.9%, Loganair with 1.6% and Auringny at 1.3% were named as the airlines with the highest percentage of delays of three hours or more to their short-haul flights.
Of the better-known airlines to perform poorly in the short-haul delay rankings operators, Thomas Cook came sixth, easyJet came 10th and Thomson Airways was 11th.
In the long-haul market, passengers travelling with Pakistan International Airlines at 4.2%, Air India at 3.7% and US Airways at 2.1% were most likely to face three-hour delays on long-haul flights.
The average delay time for a Pakistan International Airlines flight was nearly 45 minutes.
Alex Neill, Which? director of policy and campaigns, said: “Arriving at the airport to discover your flight has been delayed is incredibly frustrating, and something thousands of holidaymakers will encounter this summer.
“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.”
