Monarch Airlines has ceased trading and its 300,000 future bookings for flights and holidays have been cancelled, the Civil Aviation Authority has said.
About 110,000 customers are currently overseas and the government has asked the CAA to charter more than 30 aircraft to bring them back to the UK.
Monarch is the UK’s fifth biggest airline and the country’s largest ever to go into administration.
Customers due to fly from the UK have been told not to go to the airport.
Monarch Airlines failure: Live updates
Monarch had been in last-ditch talks with the CAA about renewing its licence to sell package holidays.
It had until midnight on Sunday to reach a deal with the aviation authority but failed to do so.
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Advice to Monarch customers, issued by CAA
Customers in the UK yet to travel: Don’t go to the airport
Customers abroad: Everyone due to fly in the next fortnight will be brought back to the UK at no cost to them. There is no need to cut short a stay
Customers currently overseas should check monarch.caa.co.uk for confirmation of their new flight details – which will be available a minimum of 48 hours in advance of their original departure time
All affected customers should keep checking monarch.caa.co.uk for more information
The CAA also has a 24-hour helpline: 0300 303 2800 from the UK and Ireland and +44 1753 330330 from overseas
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The CAA said the situation was “unprecedented”, but the 110,000 customers currently overseas would be returned home at no additional cost to them.
Dame Deirdre Hutton, chairwoman of the authority, said passengers from as far away as Tel Aviv would require repatriation and that two “rescue flights” from Ibiza had already taken off.
She asked passengers for patience, saying the CAA was having to effectively create one of the UK’s largest airlines overnight, adding: “It is a huge undertaking.”
The CAA said the “vast majority” of customers due to fly on Monday would return by the end of the day.
‘Hugely distressing’
Andrew Haines, chief executive of the CAA, said the “scale” of the operation means “some disruption is inevitable”.
He added: “We ask customers to bear with us as we work around the clock to bring everyone home.”
Transport Secretary Chris Grayling said he had ordered “the country’s biggest ever peacetime repatriation”.
“This is a hugely distressing situation for British holidaymakers abroad – and my first priority is to help them get back to the UK.”
However, 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.
By Joe Lynam, BBC business correspondent
Monarch has experienced the perfect storm of challenges in recent years.
The terror attacks in Turkey and Egypt have deprived the airline of a large chunk of its annual revenues, and forced it to compete on heavily congested traditional routes to Spain and Greece.
That has forced down prices and profits on top of weaker demand from UK travellers – for whom a less valuable pound has made travelling costlier.
The short-haul market has been described as “horrendous” by senior aviation industry figures. It has already resulted in the collapse of Air Berlin and placed huge pressure on other airlines.
Put simply, there are too many seats and not enough bums to put on them to make a profit for all major carriers.
Monarch carried 6.3 million passengers last year to 40 destinations from Gatwick, Luton, Birmingham, Leeds-Bradford and Manchester airports.
The airline, founded in 1968, employs about 2,500 people and is made up of a scheduled airline, tour operator and an engineering division.
Monarch’s owner, Greybull Capital, had been trying to sell part or all of its short-haul operation so it could focus on more profitable long-haul routes.
The airline reported a loss of £291m for the year to October 2016, compared with a profit of £27m for the previous 12 months, after revenues slumped.
How will package holiday customers be affected?
For people who booked package holidays – but have not yet flown – they will be able to apply for a refund through the Atol scheme, which refunds customers if a travel firm collapses.
“Experience suggests this will take weeks or months rather than days,” says Simon Calder, travel editor at the Independent.
He told BBC Radio 5 Live that the news was “absolutely heartbreaking” but the main thing for Monarch customers abroad was not to panic: “You will be brought home more or less on schedule.”
“There is no point in arriving at the airport on the wrong day. Just continue with your holiday,” he said.
By law, every UK travel company which sells air holidays has to hold an Atol licence. Monarch’s website says it only held the licence for package holidays, not flight-only tickets.
BBC