Britons could face up to five million plane tickets being cancelled if there is a no-deal Brexit - as airlines were today accused of failing to warn passengers.
A consumer watchdog suggested the industry is 'unwilling to give any information' about the consequences of crashing out of the EU, and should be 'more up front'.
The intervention comes after the International Air Transport Association (IATA) said up to five million bookings were 'at risk' this year if there is no deal.
The EU has said the number of flights to destinations on the continent will be kept at last year's levels, but the number of tickets issued has been expanded dramatically since then. There are also concerns that UK travellers will need to have at least six months left on their passports.
The threat of no deal has been rising as Theresa May struggles to find a way through deadlock in Parliament. Her blueprint was resoundingly rejected by MPs in an historic vote last week.
Theresa May (pictured with the New Zealand prime minister Jacinda Arden in Downing Street today) is scrambling to find a way through the Brexit deadlock
Rory Boland, editor of Which? Travel told the Times: 'The lack of warning for passengers is alarming.'
He 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.'
Mr Boland examined the 'booking journey' on UK airlines' websites, and found they do not warn travellers of the potential fallout from a no-deal Brexit.
Only Wizz Air, the Hungarian airline, has apparently warned British travellers of the position.
'Airlines don't seem to be willing to give any information ... that's what worries you the most and makes you really fear what is going to happen [in the event of no deal],' he said.
Passengers on flights that are axed should automatically be given refunds.
According to IATA research, five million extra seats are scheduled for 2019 compared to 2018 in order to meet consumer demand.
Chief executive Alexandre de Juniac said last week: 'That current flight levels will be protected even with a hard Brexit is an important assurance.'
'But with two months left until Britain leaves the EU, airlines still do not know exactly what kind of Brexit they should be planning for.
'And there is legal and commercial uncertainty over how the Commission's plan to cap flight numbers will work.
'In the small window remaining before Brexit it is imperative that the EU and UK prioritize finding a solution that brings certainty to airlines planning growth to meet demand and to travelers planning business trips and family holidays.'
Mrs May will unveil her Brexit 'Plan B' today - trying to win over Tory Eurosceptics and the DUP by securing more concessions from the EU.
The PM has been scrambling to find a way through after her deal was humiliatingly crushed in the Commons last week.
But in a conference call with Cabinet last night Mrs May effectively ruled out trying to find a cross-party consensus - instead making clear she will focus her efforts on bringing Brexiteer rebels and the DUP back onside.