The British government has asked the Civil Aviation Authority (CAA) to charter more than 30 aircraft to bring back to the UK about 110,000 Monarch customers currently overseas, the CAA said.
Tough competition has been pressuring European airlines and driving consolidation, with Air Berlin and Alitalia filing for insolvency this year and seeking new investors for parts of their businesses.
"Mounting cost pressures and increasingly competitive market conditions in the European short-haul market have contributed to the Monarch Group experiencing a sustained period of trading losses," said Blair Nimmo, a partner at KPMG and a joint administrator of Monarch Airlines Ltd and Monarch Travel Group Ltd.
The collapse will be a headache for some of the world's largest leasing companies, which financed its current fleet of 36 mainly Airbus jets, and for Boeing Co, which has sold the airline 32 of its 737 MAX aircraft. None of the planes has yet been delivered.
Although relatively small compared to Europe's leading scheduled carriers, Monarch had been a regular hotspot in the global battle for market share between planemakers as it shifted its loyalties between Airbus and Boeing.
Boeing secured Monarch's agreement to revert back to its jets in 2014 following a fierce contest against Airbus and Bombardier.
"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 Chief Executive Andrew Swaffield told employees in a message.
"And many suppliers will suffer hugely as a result of our insolvency - for which I am equally sorry."
Monarch said its companies that entered into administration include Monarch Airlines, MonarchHolidays Ltd, First Aviation Ltd, Avro Ltd and Somewhere2stay Ltd.
Monarch had previously said it was talking to potential partners after a report that parts of its short-haul network would be sold.
(Reporting by Subrat Patnaik in Bengaluru; Alistair Smout in London and Tim Hepher in Paris; Editing by Amrutha Gayathri)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)