The collapse of airline Flybmi has left passengers who booked tickets with them stranded abroad, as it emerged the firm advertised for flights just a day before entering administration.
On Friday, the company was posting offers for trips to Munich and a link to their site.
Their tweet read: "It's the height of the winter-sports season, so why not fly to Munich and enjoy world-class winter-sports resorts."
Rory Boland, Which? Travel Editor, said the situation was "terrible news" for Flybmi passengers, adding: "Some customers have claimed that tickets were being sold in the hours before the airline went bust, knowing full well those tickets would never be honoured, and passengers will rightly be outraged if this is proved to be the case."
He said customers should be exploring their options for refunds.
One passenger, Hannah Price, 23, received an email just before midnight on Saturday where she was told that her flight home had been cancelled.
The Bristol business owner said she first learned of the cancellations from a news app on her phone rather than the flight company.
She said: "I was left in the dark at first. Luckily for me my flight to Bristol operated on a shared ticket system. So I'm able to return.
"I travel to Belgium to buy materials for my clothing brand - if Brussels Airlines hadn't stepped in it would have been catastrophic.
"There was no cancellation email, and I had already booked in. It was a grey area where I stood, but my flight has been rebooked for Monday."
Hannah said that due to a previous agreement between the companies, Brussels Airlines would honour her booking for next month.
She said: "I've learnt that my next flights will take seven hours due to stopovers, this is a flight time that usually takes 50 minutes."
Richard Edwards, 47, from Midhurst, in West Sussex was about to board a flight to Munich when Flybmi stopped their services.
He said: "My family and I were some of the lucky ones. There were people being told not to even bother turning up at the airport in the end.
"We got to Bristol for our flight, due to take off at 12.10pm. We got through security and walked to the gate, then I heard an announcement.
"They just said the flight had been cancelled due to operational reasons - and for us to collect our luggage.
"We were ferried out of the airport and into a taxi - there were 14 taxis for our flight all headed for Heathrow.
"There's no talk of compensation yet. We were lucky enough to get to Munich in the end, albeit significantly delayed.
"Lufthansa got us out to Munich in the end. We still haven't heard anything about our return flight though - so we could be stranded."
Another customer, Julian Cassell, wrote online: "I have two tickets for Munich in July if anyone wants them. I will sell for face value."
And civil engineer Danny McLaughlin, who lives along the Irish border, said he had 14 flights booked through his job over the next seven weeks between City of Derry Airport and London Stansted.
He said: "I'm a bit stunned to be honest with you. I know airlines and things have been saying this for quite a while but it just came as a bolt out of the blue."
A spokesperson for Flybmi said: “It is with a heavy heart that we have made this unavoidable announcement today.
"The airline has faced several difficulties, including recent spikes in fuel and carbon costs.These issues have undermined efforts to move the airline into profit.
"Current trading and future prospects have also been seriously affected by the uncertainty created by the Brexit process, which has led to our inability to secure valuable flying contracts in Europe.
“Additionally, our situation mirrors wider difficulties in the regional airline industry which have been well documented.”
Brian Strutton, General Secretary of the British Airline Pilots Association, said: “The collapse of FlyBMI is devastating news for all employees.
"Regrettably BALPA had no warning or any information from the company at all.
"Our immediate steps will be to support FlyBMI pilots and explore with the Directors and administrators whether their jobs can be saved.”