Which? looked at the policies of 14 airlines and found that nine regularly overbook their aircraft, in order to minimise the effect of no-shows. Of these, four - Air Transat, EasyJet, United Airlines and Virgin Atlantic - say they'll offer compensation or extra benefits to passengers that get bumped, such as vouchers or cash; EasyJet, for example, gives £100 to people who volunteer for a later flight.
However, Aer Lingus, Jet Airways, Lufthansa and Swiss Airlines offer only the standard EU compensation. Under the EU's Denied Boarding Regulations, passengers are entitled to €250 for flights under 1,500 kilometres, €400 Euro for all longer flights within the EU, and for all other flights between 1,500 kilometres and 3,500 kilometres; it's €600 for all other long-haul flights.
However, these sums are halved if the final arrival time is within two hours for short haul flights, within three hours for medium haul flights and within four hours for long haul flights.
A spokesman for the Civil Aviation Authority says that since the introduction of these rules, complaints about overbooking have fallen dramatically. "It's something that in the past has been a reasonably large issue, but we don't get so many calls about it now," he says. "Denied boarding is probably third or fourth on the list of complaints we receive."
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Passengers that aren't prepared to accept being bumped under any circumstances should stick to flying with Jet2, Monarch, Ryanair, Thomas Cook Airlines or Thomson Airways, all of which have a policy of never overbooking.
But with £100 on offer, many passengers are perfectly happy to wait for the next flight.
"We quite often have the impression that people hate being in an airport, and really don't want to spend any longer there than they have to," says the CAA spokesman. "But for the offer of a few hundred euros, many people are quite happy to do it - they'll just have a drink and get a few things in duty-free."