Around 1.3 million air passenger journeys were estimated to be subject to a severe delay last year, according to new Which? analysis.
The consumer organisation found that, in the year to June 2018, over 13,000 flights flying to or from UK airports were delayed by at least three hours.
That is equivalent to nearly three and a half thousand passenger journeys a day.
Norwegian (2.4 per cent) Thomas Cook (1.8 per cent) and TUI (1.6 per cent) had the highest percentage of severe long-haul delays.
Thomas Cook saw the percentage of long-haul flights delayed by three hours increase significantly (from 1.1 per cent in the year to June 2017) – with the latest figures revealing that over 7,500 passenger journeys could be affected.
Thomas Cook (1.2 per cent), TUI (1.1 per cent), and Saudi Arabian Airlines (one per cent) had the highest proportion of delays of at least three hours for medium-haul flights.
The figures for TUI and Thomas Cook represent around 38,000 passenger journeys combined.
Icelandair (1.7 per cent), Aurigny (1.6 per cent) and TUI (1.4 per cent) experienced the highest proportion of severely delayed short-haul flights.
The Icelandic flag carrier has seen a three and a half-fold increase in proportion of three-hour delays in the last year.
Which? also found that the bulk of severely delayed flights could be attributed to easyJet (2,618), Ryanair (1,868) and British Airways (1,668).
Although the three-hour delay rates of the airlines were close to the industry average, the high volume of passengers they carried still meant that more than 630,000 passenger journeys were severely delayed.
Under current EU regulations, passengers are entitled to compensation if delayed by more than three hours when flying from the UK or with an EU airline to an EU airport.
Airlines are only exempt from paying out if they can prove that the delay or cancellation was caused by ‘extraordinary circumstances’ such as extreme weather conditions or airport strikes.
According to the Civil Aviation Authority, industrial action taken by an airline’s own employees, such as the Ryanair pilot strikes that took place this summer, does not count as an extraordinary circumstance and passengers can claim compensation if a flight is severely delayed or cancelled for this reason.
Alex Neill, Which? managing director of home products and services, said: “Severe delays can be a complete nightmare and totally wreck a long-awaited trip abroad, especially if it means you’re stuck in an airport terminal for hours on end.
“Passengers are often entitled to compensation when airlines get it wrong.
“It is vital that automatic compensation is introduced across the industry so that people no longer have to jump through hoops to get what they are owed.”
Which? believes that it is time for airlines to start automatically compensating eligible passengers for delayed and cancelled flights, as the current process can be complicated and time consuming.
This can lead to passengers turning to claims management companies to handle their compensation claim – but previous Which? research has found that these businesses can take up to a 40 per cent of pay-outs.
Until automatic compensation is introduced, passengers can use the Which? guide to make a claim.
This is a free tool, so people who successfully claim compensation won’t have to share any of their pay out.