The carrier is the biggest British airline to ever cease trading
Shares in Irish carriers Ryanair and Aer Lingus owner IAG have gone up after one of its rival UK-based airlines ceased trading.
Monarch has confirmed it has ceased trading and now entered administration.
As of October 2nd 2017, all future holidays and flights provided by Monarch and its related companies have been cancelled and are no longer operating.
The UK's Civil Aviation Authority (CAA) says: "This is an unprecedented situation and because there are up to 110,000 passengers abroad".
The British government has asked the CAA to coordinate flights back to the UK for all Monarch customers currently overseas.
It says these new flights will be at no extra cost to consumers affected.
More than 30 aircraft have been chartered to bring customers back to the UK over the next fortnight.
Some 300,000 future bookings have been axed.
Monarch chief executive, Andrew Swaffield, blamed a drop in profits on "the closure, due to terrorism, of Sharm-El- Sheikh and Tunisia and the decimation of Turkey".
He added: "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."
Monarch secured a 24-hour extension to its tour operator's licence on Saturday night, but it proved to be a temporary stay of execution amid uncertainty over the airline's finances.
The firm, which has its headquarters at Luton Airport in London, employs around 2,750 people and is the biggest UK airline to ever cease trading.
Additional reporting: IRN