Five million airfares could be cancelled on the tarmac in the event of a "no-deal Brexit".
However, airline operators have been accused of being "unwilling to produce information" about the consequences of an uncoordinated United Kingdom exit from the European Union.
The International Air Transport Association (IATA) said as many as five million tickets were "at risk" should negotiations not produce a deal.
Rory Boland, the editor of UK consumer rights magazine Which? said "the lack of warning for passengers is alarming".
In an interview with The Times of London, Boland said "It is the job of airlines to tell passengers what is likely to happen. If they were more upfront it would give passengers the information they need to protect themselves."
Examining the booking process on websites belonging to UK airlines Boland's publication found no warning of the potential risks to passengers' journeys, even when booking flights on or after March 29, 2019 – the date when the UK is scheduled to cease to be a member of the EU, with or without a deal being negotiated.
What happens if your flight gets axed by Brexit?
Passengers with tickets that are cancelled by Brexit should automatically be given refunds.
IATA chief executive Alexandre de Juniac said: "That current flight levels will be protected even with a hard Brexit is an important assurance."
A number of flights have been safeguarded by a cap agreed on during EU negotiations, which means for now air traffic will be protected at 2018 levels.
Although the IATA spokesperson admitted that there is still "legal and commercial uncertainty over how the Commission's plan to cap flight numbers will work."
The Association of British Travel Agents, the UK trade body for travel agents said: "The European Commission and the UK Government have both said that, even in a no-deal scenario, flights will continue to operate between the UK and the EU."
While the UK and EU have agreed to protect flights in UK airspace by capping numbers at 2018 levels, IATA has estimated an extra five million seats have been scheduled in 2019 to cover projected travel demands.
This leaves some 15,000 flights and their passengers with uncertainties whether their travel plans will go ahead.
What the airlines are saying
Thomas Cook Airlines told the Daily Mail that its flight schedules will not be affected after making no plans to increase its summer flight capacity over 2018 levels. The Thomas Cook website also has an extensive guide for customers.
German airline Tui assured passengers that it "actively prepares for all possible scenarios", insisting in a statement that it will continue to "operate our holidays to the EU and our other amazing destinations as planned."
A spokesperson for the budget operator EasyJet said: "EasyJet remains confident that flights will continue, even if the outcome is a no-deal Brexit, as both the European Union and the UK have set out their plans to ensure that connectivity is not be disrupted."
Which? found Hungarian owned Wizz Air to be the only operator actively warning UK-bound passengers about the uncertainty facing their travels.
The Hungarian airline issued a warning to UK passport holders on their website.
It said: "The UK government is working with partners in the travel and transport industry to make sure people who may be affected are aware of the new rules.
"It is an individual's responsibility to make sure they have the correct documentation for travel, including a passport that complies with the validity rules of whichever country they are travelling to."
Airline catering: A taste of what's to come
Gate Gourmet, a catering company that provides meals for 20 UK airlines has begun stockpiling inflight meals. The company, which is the world's biggest caterer for the aviation industry, is worried the impact of a no-deal Brexit could leave flights without food.
Many of the Swiss company's ingredients, which come from the EU through the common market, could be affected by border negotiations.
In an interview with Bloomberg, Stephen Corr, the company's managing director for Western Europe, said: "Companies could be in difficulty if they haven't prepared themselves and ensured a continuity of supply."
Corr estimated current frozen stock at the storage facility in Peterborough could cover between seven to ten days disruption.
Gate Gourmet's clients include British Airways and Easyjet.