Around 300,000 holiday bookings have been cancelled after the collapse of Monarch Airlines.
The UK's biggest peacetime repatriation operation is also under way to return 110,000 of the firm's customers in the wake of its administration, which was confirmed in the early hours of today.
The Civil Aviation Authority (CAA) said it had been asked by the Government to charter more than 30 aircraft to bring the passengers back to the UK after the airline's board called in administrators KPMG in the early hours of Monday morning.
The collapse - the largest to hit a UK airline - has left some 300,000 future bookings cancelled and customers have been told to keep away from airports as there will be no more flights.
Passengers are urged to check a dedicated website for advice.
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 will 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 delay 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 in 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.