"Due to the significant scale of the situation, some disruption is inevitable, but the Civil Aviation Authority will endeavor to get people home as close as possible to their planned dates," the aviation authority said in a statement.
Describing the repatriation plan, British Transport Secretary Grant Shapps said dozens of charter planes, from as far afield as Malaysia, had been hired to fly customers home free of charge. He said hundreds of people were staffing call centers and airport operations centers.
"The task is enormous, the biggest peacetime repatriation in UK history. So there are bound to be problems and delays," he said.
A website set up by the aviation authority to aid the firm's customers crashed shortly after the company collapse was announced.
Unions representing the Thomas Cook staff had urged the British government to intervene to prop up Thomas Cook.
Most of Thomas Cook's British customers are protected by the government-run travel insurance program, which makes sure vacationers can get home if a British-based tour operator fails while they are abroad.
Thomas Cook, which began in 1841 with a one-day train excursion in England and now operates in 16 countries, has been struggling over the past few years. It only recently raised 900 million pounds ($1.12 billion), including receiving money from leading Chinese shareholder Fosun.
An estimated 1 million future travelers will find their bookings for upcoming holidays cancelled. They are likely to receive refunds under the terms of the government's travel insurance plan.
In May, the company reported a debt burden of 1.25 billion pounds and cautioned that political uncertainty related to Britain's scheduled departure from the European Union at the end of October had hurt demand for summer holiday travel. Heat waves over the past couple of summers in Europe have also led many people to stay at home, while higher fuel and hotel costs have weighed on the travel business.
The company's troubles were already affecting those traveling under the Thomas Cook banner.
A British vacationer told BBC radio on Sunday that the Les Orangers beach resort in the Tunisian town of Hammamet, near Tunis, demanded that guests who were about to leave pay extra money for fear it wouldn't be paid what it is owed by Thomas Cook.
Ryan Farmer, of Leicestershire, said many tourists refused the demand, since they had already paid Thomas Cook, so security guards shut the hotel's gates and "were not allowing anyone to leave."
It was like "being held hostage," said Farmer, who is due to leave Tuesday. He said he would also refuse to pay if the hotel asked him.
The Associated Press called the hotel, as well as the British Embassy in Tunis, but no officials or managers were available for comment.