The UK's biggest ever peacetime repatriation effort is underway following the collapse of holiday firm Thomas Cook.
The programme, named Operation Matterhorn, is expected to last until Sunday 6 October and will rescue 150,000 stranded customers - almost twice the number brought home after Monarch Airlines failed in 2017 - at an estimated cost of £100m.
The debt-ridden company entered compulsory liquidation in the early hours of the morning after it failed to secure an extra £200m needed to keep it afloat following talks with creditors and the government.
Around one million customers who had travel booked in the coming months have been told not to go to the airport, as all bookings, including flights and holidays, have been cancelled.
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Peterborough-based Thomas Cook had a total of 600,000 global customers in-resort at the time of its demise though some European subsidiaries, including Condor, are currently unaffected as negotiations over funding continue.
The company's planes that were operating overnight were grounded after they reached their destinations while 21,000 staff, including 9,000 people in the UK, are set to lose their jobs at a time when the wider travel industry is struggling.
Workers have been told they will have to apply for their wages via the Official Receiver.
Peter Fankhauser, Thomas Cook's chief executive, said the tour operator's demise was a "matter of profound regret", apologising to all those affected.
He said executives had "worked extensively" in an effort to rescue the 178-year-old travel company, adding: "Although a deal had been largely agreed, an additional facility requested in the last few days of negotiations presented a challenge that ultimately proved insurmountable."
He added: "I would like to apologise to our millions of customers, and thousands of employees, suppliers and partners who have supported us for many years."
The Civil Aviation Authority (CAA) has mobilised more than 40 aircraft from airlines including British Airways and easyJet to bring 150,000 Thomas Cook customers home to the UK.
The first such rescue flight left New York's JFK for Manchester with 300 people on board.
A total of 16,000 were expected to have returned by the end of the day.
Richard Moriarty, the CAA chief executive, said: "We have launched, at very short notice, what is effectively one of the UK's largest airlines, involving a fleet of aircraft secured from around the world.
"The nature and scale of the operation means that unfortunately some disruption will be inevitable. We ask customers to bear with us as we work around the clock to bring them home."
The CAA's policy director Tim Johnson told Sky News: "We expect to run around 1000 flights from the 55 destinations that Thomas Cook served.
"This is a huge effort, were working very closely with the government and Foreign Office in particular...But we're particularly appreciative and grateful for the support of a number of Thomas Cook employees who have clearly received this very sad news but they're going to be working with us over the coming few weeks to really help us manage this repatriation exercise."
Thomas Cook had been one of the world's oldest and largest travel companies, established in 1841 by a cabinet maker who organised a day trip for temperance movement supporters.
According to its website, the group employed 21,000 people in 16 countries operating 105 aircraft - the vast majority of them leased - and 200 own-brand hotels and resorts. It had 550 stores in the UK - many of them loss-making.
It was hit hard by online competition, a changing travel market, terrorist attacks in destinations such as Tunisia and last year's European heatwaves. People postponing holidays due to Brexit uncertainty and higher fuel costs also weighed heavily on the company.
Business Secretary Andrea Leadsom has written to the Insolvency Service to ask them to "fast-track" their investigation into the circumstances surrounding Thomas Cook going into liquidation.
Rival holiday firm TUI saw its shares surge on news of Thomas Cook's collapse - rising 8% in early trading and closing the day up more than 6%, while easyJet ended the day up 4.6% and Ryanair at just under 1%.
Thomas Cook had a rescue deal in place with investors but faced a last-minute demand from its lenders that it have more funds on standby to see it through the tougher winter months.
PM Boris Johnson said a government bailout would have created a "moral hazard" by encouraging other companies to take unnecessary risks.
Labour argued ministers should have stepped in through the purchase of an equity stake. The party called on bosses to repay their bonuses.
One investor, Fosun Tourism Group, the owner of Club Med, had committed £450m to Thomas Cook under the plan to save the business.
It said in a statement: "Fosun is disappointed that Thomas Cook Group has not been able to find a viable solution for its proposed recapitalisation with other affiliates, core lending banks, senior noteholders and additional involved parties.
"Fosun confirms that its position remained unchanged throughout the process, but unfortunately other factors have changed.
"We extend our deepest sympathy to all those affected by this outcome."
Thomas Cook's German airline Condor is continuing to trade while the German government considers a request for an emergency bridging loan. Several other European-based subsidiaries are also unaffected at this time.