Tens of thousands of Britons are stranded abroad after the country's third largest package holiday group went into administration.
The XL Leisure Group, which operates XL airlines, flies to 50 destinations, mainly in the Mediterranean.
All its flights have now been cancelled and its aircraft grounded.
The Civil Aviation Authority (CAA) said 85,000 people could be stranded abroad and 200,000 have made advance bookings with the company.
David Clover, a spokesman for the CAA, said it was making arrangements to help customers of the four tour companies within the XL group.
"In respect of people who are currently abroad we're making arrangements and working very closely with the travel industry to organise repatriation flights.
"Clearly though, with XL Airways no longer operating, we're having to bring in substitute aircraft to bring people home."
He said package deals are covered by the CAA's Air Travel Organisers' Licensing (ATOL) scheme and those customers will be offered repatriation flights or their money back if they have an advance booking.
However, those who booked directly with the airline or XL.com - who are in the minority according to the CAA - will face a fee.
He advised those with future flights to check their insurance policies, and with their banks or credit card companies about refunds.
An XL flight from Orlando to Gatwick was grounded shortly before take-off. A source at the airport said accommodation was being found for the "distressed" passengers. An earlier XL flight bound for Manchester left as planned.
The group is the latest travel business to face financial difficulties, as the industry struggles with high fuel costs and an economic downturn.
"As the travel industry matures in Europe, there was always going to be pressure on those operating in the mid-market," said Lastminute.com chief executive Ian McCaig.
"You don't have enormous scale or specialism, so there was always going to be pressure. Economic conditions have really just accelerated that process in the case of XL."
An XL pilot, who did not wish to be named, told BBC Radio 4's Today programme he was "completely shocked" by the news the company had gone bust.
He said he was only told in the early hours of Friday morning, and blamed the demise on the rising cost of fuel.
The XL group, which is based in Crawley, West Sussex, runs an airline and owns several travel companies, including Travel City Direct, Medlife Hotels Limited, The Really Great Holiday Company, Freedom Flights and Kosmar Holidays.
The group, which carried 2.3 million passengers last year, has 1,700 employees worldwide.
A statement on the XL group's website said: "The companies entered into administration having suffered as a result of volatile fuel prices, the economic downturn, and were unable to obtain further funding."
Rival holiday firm TUI warned that rising fuel costs meant that "airlines with less than robust business models" - such as XL and Futura - were now failing.
It warned that the Government should take steps to ensure all holiday companies must belong to the Atol scheme. which offers package holiday makers financial protection.
The company flies mainly from bases at Gatwick, Manchester and Glasgow airports.
Travel writer Simon Calder said XL had 21 aircraft and flew to the Caribbean, Mediterranean, North Africa and North America, from airports across the UK.
He said: "There are still going to be tens of thousands, maybe hundreds of thousands of people...who find that they are either stranded abroad, or find that they are booked to travel in the next days, weeks, months, and they simply won't be going anywhere, I'm afraid."
Bob Atkinson, of the price comparison website Travel Supermarket said XL's troubles would be a blow for the travel trade.
He said: "They are a very large operator and this will send serious shock waves through the industry.
XL's problems mean West Ham football club may have to find a new sponsor
"And what it's going to do more than anything, it's going to highlight how precarious the airline industry is at the moment."
Jim Duwaine, from Portsmouth, said he was given the news when he arrived at Gatwick where he had been due to catch an early morning flight to Menorca.
He said: "Absolutely devastated. Got up at midnight planning on going on holiday, but got let down, unfortunately. We're here, just trying to get some other flights, but it's not looking good. I think everyone else has got the same idea."
XL is the current kit sponsor of West Ham United football club.