Video report by Consumer Editor Chris Choi
Monarch Airlines has ceased trading with immediate effect, meaning all flights and holidays have been cancelled and will not be rescheduled.
The Civil Aviation Authority (CAA) has begun a programme to bring 110,000 Monarch customers back to the UK, while 300,000 future bookings have been cancelled.
The effort to return passengers to the UK will be the country's biggest peacetime repatriation operation.
The Government has warned passengers to expect disruption and delays as it works to ensure there are enough flights.
What has caused Monarch's collapse?
Monarch said in a statement to staff the "root cause" of the airline's closure was "due to terrorism, of Sharm-El-Sheikh and Tunisia and the decimation of Turkey" which saw profits plunge in recent years.
Monarch CEO Andrew Swaffield said despite efforts to cut costs a review found attempts to save the company was "pretty much out of our control".
Monarch's profit in 2015.
Monarch's loss in 2017.
Monarch's projected loss in 2018.
Mr Swaffield apologised to all Monarch Airlines and Monarch Travel Group employees, telling them to "hold their heads up high and be proud of what you achieved at Monarch".
How will the repatriation operation work?
Dozens of planes will bring those whose flights have been cancelled home within the next fortnight, after the airline failed to renew a crucial licence.
Nearly 12,000 passengers were brought home on Monday, the Civil Aviation who are organising the flights, said.
Foreign Office staff will be at affected airports to assist vulnerable British travellers with specific needs.
Holidaymakers abroad do not need to cut short their stays to return home and will incur no extra costs over the changes.
Transport Secretary Chris Grayling said the repatriation flights were "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," he said.
"Nobody should underestimate the size of the challenge, so I ask passengers to be patient and act on the advice given by the CAA."
How many jobs will be lost?
Monarch employs around 2,750 UK-based staff - of which 1,858 have now been made redundant, administrators KPMG said.
Ninety-eight of these were employed by Monarch Travel Group, while 1,760 were employees of Monarch Airlines.
The company, whose headquarters are at London Luton Airport, was founded in 1968 and was the UK's fifth-largest airline.
It also operates from London Gatwick, Manchester, Birmingham and Leeds Bradford to more than 40 destinations around Europe and further afield.
What has Monarch said?
Monarch CEO Andrew Swaffield's statement to staff included an apology to holidaymakers as he accepted the scale of disruption was huge.
"Millions of customers have flown and holidayed with Monarch over the last 50 years," he said.
"I am so sorry that thousands now face a cancelled holiday or trip, possible delays getting home and huge inconvenience as a result of our failure.
"We are working with the joint administrators and the CAA to do everything we possibly can to help minimise disruption where we can, but are under no illusion as to the problems this will cause.
"And many suppliers will suffer hugely as a result of our insolvency – for which I am equally sorry."