Holidaymakers are being left in the dark over the consequences of the UK leaving the EU without an aviation deal, a consumer group has claimed.
Which? called on travel firms to do more to highlight the possible financial implications for consumers if no agreement to continue flights is reached.
It is advising customers who have booked holidays taking place after the Brexit date of March 29 next year to check cancellation and refund policies.
The single market for aviation, created in the 1990s, means there are no commercial restrictions for airlines flying within the EU.
Ryanair boss Michael O'Leary has repeatedly warned that flights could be grounded if an aviation deal is not reached as part of the Brexit negotiations.
Which? contacted the UK's five biggest travel companies to find out what they are doing to inform people about the possible impact on their holidays.
Tui, Jet2 and On the Beach "failed to provide any reassurance that any information would be communicated upfront", the consumer group reported.
It noted that Thomas Cook has amended its terms and conditions to state that it will not provide compensation or reimburse expenses if it changes the circumstances of a booking due to airspace closures.
The firm encourages customers to take out travel insurance to cover consequential losses.
Expedia told Which? it believes passengers will be entitled to compensation in the event of disruption, although it is not yet selling post-March 2019 holidays.
Peter Vicary-Smith, chief executive of Which?, said uncertainty for holidaymakers is "one of the many issues affecting people's everyday lives that need to be resolved".
He added: "We want to work with Government and businesses on issues such as this in order to deliver a Brexit that puts consumers first."
A spokeswoman for travel trade organisation Abta said: "Package holidays will continue to be covered by regulations which give holidaymakers the right to an alternative holiday, if available, or a refund in the event of changes caused by extraordinary circumstances."
Tim Alderslade, chief executive of Airlines UK, the trade association for UK carriers, said a deal to maintain open aviation arrangements is "readily achievable".
He went on: "Just as they do today, airlines will continue to comply with all legal requirements, paying compensation quickly when it is due and making it easy for passengers to claim."