Refunding 360,000 ATOL-protected customers with holidays booked with Thomas Cook could take up to 60 days to complete, the aviation regulator has warned.
The Civil Aviation Authority said direct debit customers could expect their refunds to be processed within 14 days but added others would face a wait of up to 60 days.
It said it would launch an online refund system from October 7, which would provide "an electronic refund form".
The tour operator’s collapse means 22,000 jobs are at risk worldwide, including 9,000 in the UK.
The Civil Aviation Authority is already undertaking “the biggest-ever peacetime repatriation” of UK residents as it brings 150,000 holidaymakers home.
It is now turning its attention to a refund programme three times larger than any it has managed before, the regulator said.
“For around 100,000 bookings made by direct debit we hope to refund these within the next 14 days. Refunds of bookings made by other payment methods will take longer as we do not yet have all of the information we need from Thomas Cook,” Richard Moriarty, chief executive of the Civil Aviation Authority, said.
On September 23, insolvency practitioners from KPMG and AlixPartners were appointed to Thomas Cook Group, including its tour operator and airline, after the 178-year-old travel company ceased trading.
KPMG confirmed Blair Nimmo, Jim Tucker and David Pike have been appointed by the court as special managers to the retail division of Thomas Cook.
Meanwhile, Simon Appell, Alastair Beveridge, Dan Imison and Ben Browne of AlixPartners were appointed to assist the official receiver in the liquidation of 19 different Thomas Cook companies.