It was the oldest travel company in the world — a behemoth that ran its own hotels and flights, serving 19 million travelers every year and employing 21,000 people.

So when the Manchester, England, headquartered firm Thomas Cook collapsed on Monday after a last-ditch effort failed to resolve the billions in debt it had accumulated, hundreds of thousands of travelers worldwide were suddenly stranded, prompting an aviation regulator in the United Kingdom to scramble to get them home.

The United Kingdom's Civil Aviation Authority dubbed the hastily put-together plan "Operation Matterhorn" and said it was the largest peacetime repatriation effort in British history, a complex effort to return 150,000 Britons.

The global operation is relying on about 45 aircraft operated by at least six different airlines. A Civil Aviation Authority press officer cautioned Monday evening that the exact number of jets could not be confirmed because the situation was "constantly changing."

The fleet will return British citizens from as many as 18 countries around the world. CAA chief executive Richard Moriarty pleaded for patience from those stranded.

“We have launched, at very short notice, what is effectively one of the U.K.'s largest airlines."

“We have launched, at very short notice, what is effectively one of the U.K.'s largest airlines, involving a fleet of aircraft secured from around the world. The nature and scale of the operation means that unfortunately, some disruption will be inevitable. We ask customers to bear with us as we work around the clock to bring them home,” he said in a statement.

In total, Thomas Cook has 600,000 customers abroad, including the British citizens, according to Reuters. Its collapse meant that travelers who were in the midst of pre-planned vacations to Turkey, Greece, Tunisia and other countries found out on Monday that their flights home had been canceled, and those who had vacations in the future they had already paid for discovered those had been nixed, too.

The CAA acknowledged the news "will be very distressing for its customers and employees" and said its repatriation program, launched at the request of the U.K. government, will go through Sunday, Oct. 6. Information on repatriation flights was available online, as were details about the Air Travel Organizer's License, a program that gives financial protection to customers who have purchased vacation packages sold by British travel businesses.

Transport Secretary Grant Shapps vowed to make the operation run as smoothly as possible.

"We're going to get you home," Shapps said in an interview with ITN. "Please be patient. This is a huge operation — the biggest repatriation in history. But we will get you home and make sure you enjoy the rest of your holiday."

Travel woes proliferated on social media on Monday: customers who were suddenly forced to leave their hotels, honeymoons canceled, and hours-long lines to get on flights.

Many travelers were understanding of the sudden switch in plans, though, and said they were more worried for the tens of thousands of Thomas Cook employees who had lost their jobs than about their vacations.

"My honeymoon has gone down the drain as we had booked with #thomascook but I don’t care, I’ll get refunded. I care about the employees who have lost their jobs with no warning," tweeted one.

Founded in 1841, Thomas Cook was struggling to reduce a debt pile of $2.1 billion that ballooned during the 2016 coup attempt in Turkey, one of its top travel spots, and the 2018 European heatwave.

It is not the first British travel firm to fail in recent years. Two years ago, Monarch, a British carrier and tour operator, collapsed, stranding more than 100,000 passengers abroad and prompting the government to bring them home. At the time, it was the biggest airline collapse in U.K. history.

The name "Operation Matterhorn" was also used during World War II, when Allied forces attacked Japanese forces, mostly in India and China.