The CEO of British travel firm Thomas Cook said it was a "deeply sad day" and apologised "to millions" after the cash-strapped travel company went into compulsory liquidation, leaving 600,000 holiday-makers stranded worldwide.

The UK Civil Aviation Authority has confirmed the tour operator, one of the oldest and largest in the world, had "ceased trading with immediate effect" after last-minute negotiations failed to save it.

There are about 600,000 Thomas Cook customers currently on holidays, including 150,000 from the UK, news.com.au reports.

An emergency operation codenamed Operation Matterhorn will aim to bring home the 150,000 British tourists in what is believed to be the biggest peacetime repatriation operation in British history, the BBC reports.

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Empty aircraft have already begun flying overseas to bring Thomas Cook customers home. Customers who have not yet left the UK are being told not to go to the airport.

Thomas Cook, which at 179 years old is one of the world's oldest travel providers, runs airlines, cruises, hotels, resorts and travel packages for 19 million customers a year in 16 countries. It also employs about 22,000 staff worldwide.

Its recent financial strife has been attributed to tough online competition, the high price of jet fuel and concerns over Brexit.

Chinese firm Fosun had agreed to backing a $1.73 billion refinancing but Thomas Cook's banks ordered it to cough up an extra $398 million as a contingency to see it through winter. It was unable to secure the extra funds before a deadline today.

In a statement, Thomas Cook said the company's board "concluded that it had no choice but to take steps to enter into compulsory liquidation with immediate effect".

"It is a matter of profound regret to me and the rest of the board that we were not successful," the company's chief executive Peter Fankhauser said.

Thomas Cook collapsed after failing to secure rescue funding. Photo / AP
Thomas Cook collapsed after failing to secure rescue funding. Photo / AP

"I would like to apologise to our millions of customers, and thousands of employees, suppliers and partners who have supported us for many years. Despite huge uncertainty over recent weeks, our teams continued to put customers first, showing why Thomas Cook is one of the best-loved brands in travel.

"Generations of customers entrusted their family holiday to Thomas Cook because our people kept our customers at the heart of the business and maintained our founder's spirit of innovation.

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"This marks a deeply sad day for the company which pioneered package holidays and made travel possible for millions of people around the world."

Earlier, Flightradar24 captured dozens of Thomas Cook aircraft flying back to Britain as fears grew for a collapse, which was eventually confirmed after midnight in the UK.

British customers of Thomas Cook would be protected under Atol, the UK government's financial protection scheme for package holidays, however the Financial Times noted the collapse of Thomas Cook would be the scheme's biggest test yet.

$65K WEDDING IN TATTERS

Chloe Hardy, from Leicestershire, UK, and her partner spent about $65,000 on a destination wedding on the Greek island of Zante next month.

They booked the wedding package with Thomas Cook in June last year and now have no idea what will happen.

"We are due to fly out on Thursday," Ms Hardy told The Sun.

"Our wedding is costing almost £12,000 ($23,858) with things we have bought here, the wedding package, everything we have organised with the resort wedding co-ordinator."

Ms Hardy said she and her partner had arranged to fly 33 relatives to the Zante for the wedding.

Chloe Hardy and her partner. Photo / Facebook
Chloe Hardy and her partner. Photo / Facebook

She said the bookings were protected under Atol but the cancellation would still be devastating.

"We are unsure if we will be able to fly," she said.

"I have booked three weeks' leave from work and there's no guarantee that I will be able to get time off if I had to re-book."

Another couple, Sami Ryder and Dean Turgeon, said they'd booked a Thomas Cook wedding package in Santorini in September.

"The wedding package includes wedding venue, celebrant to marry us, bouquets, et cetera, and we have around 30 to 35 guests flying out with us," Ms Ryder said.

Matthew Moore and partner Aaron, from County Armagh in Northern Ireland, said they spent about $36,800 for their dream wedding in Cyprus in two weeks, with 26 guests.

"A few of the guests have been asking us whether we know what's going on but we're just telling them what we're getting told by Thomas Cook," he told The Sun.

"They're contacting us saying they're stressed and we're trying to tell them, 'Well can you understand how stressed we are?'

"If it was a normal holiday it wouldn't be so bad but it's a wedding.

"All the stress of planning a wedding is hard enough but this is causing serious anxiety and sleepless nights."

'WE'RE BEING HELD HOSTAGE'

Meanwhile tourists in Tunisia, who booked their tours with Thomas Cook, have been barricaded inside a resort over fears the travel firm will be unable to pay bills.

Thomas Cook customers staying at the Les Orangers hotel in Tunisia said they had been locked inside until they paid 6000 Tunisian dinars, about $A3350, to catch their flights home.

"We're being held hostage," British tourist Ryan Farmer told the BBC.

He said guests due to leave were being told to go to reception to pay "additional fees, obviously because of the situation with Thomas Cook".

"We've been up to the gates. They had four security guards on the gates, holding the gates closed, and were not allowing anybody to leave," he said.

Another British tourist, Chris Rutherford, said the hotel reception "told us we need to pay £4000 ($7,951) to be allowed to leave".

"They made an elderly lady who had fallen and broken her arm whilst on holiday pay £2500 ($4,969) to leave the hotel," he said.

"The Thomas Cook rep told us last night they were only dealing with people who had to leave that evening. As we are supposed to be here until Friday, we got no information and have not had any further correspondence with Thomas Cook."

The hotel gates have since been reopened, but customers fear they'll be closed again once the next crop of tourist are due to leave, the BBC reported.

The British Embassy has reportedly spoken to the hotel.