The UK's biggest peacetime repatriation operation is underway to return 110,000 Monarch Airlines customers after the airline collapsed into administration.
The Civil Aviation Authority (CAA) said it had been asked by the UK government to charter more than 30 aircraft to bring the passengers back to Britain after the airline's board called in administrators KPMG in the early hours of Monday.
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 2100 people 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.
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."
Monarch, whose headquarters are at London Luton Airport, was founded in 1968.
