Some 110,000 passengers are stranded overseas and 300,000 future bookings have been cancelled after British airline Monarch collapsed into administration.
"We are sorry to inform you that, as of 2 October 2017, all future holidays and flights provided by these companies have been cancelled and are no longer operating," said a message on Monarch's website under the logo of Britain's Civil Aviation Authority (CAA).
"This is an unprecedented situation and because there are up to 110,000 passengers abroad, the UK government has asked the CAA to co-ordinate flights back to the UK for all Monarch customers currently overseas.
"These new flights will be at no extra cost to you." Some 300,000 future Monarch bookings were also cancelled, the Press Association reported.
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Monarch's collapse, which came as it failed to have a crucial licence renewed, was the largest to hit a British airline.
The UK government's "extraordinary operation" to bring customers home - to include chartering more than 30 aircraft - was Britain's "biggest ever peacetime repatriation," Transport Secretary Chris Grayling said.
"This is a hugely distressing situation for British holidaymakers abroad - and my first priority is to help them get back to the UK."
Passengers should expect delays due to the operation, the Transport Department said.
"All Monarch flights from the UK are cancelled, do not go to UK airports," said a post on Monarch's Twitter feed.