Monarch Airlines passengers are being told not to go to airports after the airline went into administration on Monday morning.
All of its 300,000 bookings have been cancelled without replacement, while stranded passengers will be flown home on 30 charter planes in the next fortnight.
The government has asked the Civil Aviation Authority (CAA) to bring home the 110,000 people left stranded after the biggest ever collapse of a UK airline.
Monarch was forced to immediately cease all flight operations after it failed to secure a crucial licence.
Passengers are urged to check a dedicated website for advice.
The CAA said it had been asked by the government to charter more than 30 aircraft to bring passengers back to the UK, after the airline's board called in administrators KPMG in the early hours of Monday.
Administrator Blair Nimmo said Monarch, which employs around 2,100 people across its airline and tour group, had struggled with mounting costs and competitive market conditions that saw it suffer a period of sustained losses.
CAA chief executive Andrew Haines said the decision to stop trading would be "very distressing for all of its customers and employees".
"We are putting together, at very short notice and for a period of two weeks, what is effectively one of the UK's largest airlines to manage this task," he said.
"The scale and challenge of this operation means that some disruption is inevitable. We ask customers to bear with us as we work around the clock to bring everyone home."
The regulator said all Monarch customers who are abroad and due to return to the UK in the next two weeks would be flown home.
The flights will be at no extra cost to passengers and they do not need to cut short their stay, the CAA said.
New flight details will be available a minimum of 48 hours in advance of customers' original departure times.
The government has warned passengers to expect disruption and delays as it works to ensure there are enough flights to return the "huge number" of passengers.
Commenting on the "extraordinary operation", Transport Secretary Chris Grayling said: "I have immediately ordered the country's biggest ever peacetime repatriation to fly about 110,000 passengers who could otherwise have been left stranded abroad.
"This is an unprecedented response to an unprecedented situation. Together with the Civil Aviation Authority, we will work around the clock to ensure Monarch passengers get the support they need.
"Nobody should underestimate the size of the challenge, so I ask passengers to be patient and act on the advice given by the CAA."
The CAA had been expected to announce on Monday whether Monarch would be able to continue selling package holidays.
The low-cost airline and holiday company had a deadline of midnight on September 30 before its Air Travel Organiser's Licence (ATOL) expired.
The firm was granted a 24-hour extension until midnight on October 1, but that also passed without any announcement of a renewal.
Mr Nimmo said: "Mounting cost pressures and increasingly competitive market conditions in the European short-haul market have contributed to the Monarch Group experiencing a sustained period of trading losses.
"This has resulted in management appointing us as administrators in the early hours of this morning."
Customers affected by the company's collapse have been urged to check a dedicated website, monarch.caa.co.uk , for advice and information on flights back to the UK.
A 24-hour helpline is also available on 0300 303 2800 from the UK and Ireland, and +44 1753 330330 from overseas.
Monarch, whose headquarters are at London Luton Airport, was founded in 1968.
The group's engineering operation, Monarch Aircraft Engineering, is not in administration and continues to trade normally.
UK travel firms selling holidays and flights are required to hold an ATOL, which protects customers with pre-booked holidays from being stranded abroad in the event of circumstances such as the company ceasing to trade.
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