Around 410,000 customers are affected by Monarch Airlines going into administration, the Civil Aviation Authority (CAA) has said.
This includes 110,000 who are currently abroad and 300,000 with future bookings.
The Luton-based carrier, which was about to mark its 50th anniversary, went into administration early on Monday, triggering uncertainty for customers and a huge effort to get people already on holiday back to the UK.
The CAA said it had been asked by the Government to charter more than 30 aircraft to bring the passengers home, with Transport Secretary Chris Grayling calling it the "biggest peacetime repatriation" effort.
Asked how long the CAA has been planning the operation, chief executive Andrew Haines replied: "We had notification from Monarch four and a half weeks ago that there were issues they were dealing with.
"Unfortunately we didn't get final confirmation until 4am this morning and my understanding is that the board resolution to go into administration didn't take place until close to midnight on Saturday night."
The "root cause" of Monarch's reduction in revenue was terror attacks in Egypt and Tunisia as well as the "decimation" of Turkey, according to the company's boss.
Chief executive Andrew Swaffield said the airline was carrying 14% more passengers than last year for £100 million less revenue.
It was badly affected by the 2015 bombing of a Russian airliner which departed from Sharm el-Sheikh in Egypt, an attack in Sousse, Tunisia, during the same year which left 30 Britons dead and a 2016 attempted coup in Turkey.
In a letter to staff which he described as "the update I hoped I would never have to write", Mr Swaffield explained that the firm went into administration "despite our best efforts".
He said Monarch attempted to "pivot our airline from short-haul to long-haul to reduce losses", but was unable to find a "deliverable offer" from a buyer for its short-haul operations or assets.
Mr Swaffield said he was "so sorry" for causing "huge inconvenience to customers".
He went on: "We are working with the joint administrators and the CAA (Civil Aviation Authority) 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."
Mr Swaffield told his employees that they can "all hold your heads up high and be proud of what you achieved at Monarch".