Which? has joined forces with consumer groups across nine European countries to call for an end to the practice and warned airlines may be in breach of consumer law.
According to Which?, passengers who miss their outbound flight can be considered a “no show”, a clause usually “buried deep in airline terms and conditions”.
This allows airlines to cancel their connecting or return flights without refund and resell their seats.
Passengers, say Which?, usually only find out their tickets have been cancelled when they arrive at the airport for their return leg and are forced to buy another seat at “vastly inflated” prices or pay a substantial fine to use their original ticket. Which? reports some fines ranging up to €3,000.
One passenger who contacted Which? said they paid Virgin Atlantic an extra £1,354 to get home from New York - more than the £1,284 cost of her original return - after missing a flight from London whereby the airline imposed its clause.
Which? investigated 16 airlines, 11 of which were using “no show” clauses. It has since written to nine carriers, including British Airways, Virgin Atlantic and Flybe, warning them they may be in brach of the Consumer Rights Act and Unfair Terms in Consumer Contracts Directive, giving them until December 28 to reply.
Two carriers, Thomas Cook Airlines and Aurigny, have already pledged to drop their clauses.
Watchdogs in the Netherlands and Greece, meanwhile, have launched legal action against KLM and Air France as carriers with the harshest clauses.
Which? says there is also legal precedent in the UK dating to 2017 ruling against “no show” clauses.
While airlines have argued such practices combat “tariff abuse” - passengers buying returns cheaper than singles - Which? says carriers such as easyJet and Ryanair have proved proved it is possible to operate without them.
Alex Neill, Which? managing director home products and services, said: “Missing a flight because you’re stuck in traffic or on a delayed train is frustrating enough, but for the airline to then turn around and say your return journey is cancelled as well is completely unfair and unjustified.
“We don’t think there’s any good reason for a ‘no show’ clause to exist – it only works in favour of the airline. It should be removed immediately by airlines, who need to show more respect for their passengers.”
Which? has written to British Airways, Flybe, Virgin Atlantic, Emirates, KLM; Air France, Singapore, Qatar and Swiss.