Thomas Cook had secured a £900m rescue deal in August led by its largest shareholder Fosun, the Chinese company that owns Club Med.
However, a recent demand from Thomas Cook's banks, which include ROBS and Lloyds, to raise a further £200m in contingency funding in case of any shortfall over the winter holiday period put the deal in doubt.
Fosun said in a statement it was "disappointed" following news of the collapse.
"Fosun confirms that its position remained unchanged throughout the process, but unfortunately other factors have changed."
Thomas Cook boss 'distressed' at company failure
BBCCopyright: BBC
Thomas Cook's chief executive Peter FankhauserImage caption: Thomas Cook's chief executive Peter Fankhauser
Thomas Cook's chief executive Peter Fankhauser has just spoken.
He says: I would like to apologies to the many thousands of hoteliers and suppliers who have stuck with our business in the good times and bad who now face difficult financial choices of their own.
"It is deeply distressing to me that it has not been possible to save one of the best loved brand in travel."
Thomas Cook - your accommodation
Thomas Cook customers whose holidays are Atol protected do not have to pay any fees to their hotel, according to the Civil Aviation Authority.
However, it admits that it may have to relocate holidaymakers to other accommodation for the duration of their stay.
Transport Secretary Grant Shapps announced that the government and UK's Civil Aviation Authority has hired dozens of charter planes to fly customers home free of charge.
On Sunday, empty aircraft had already started to be flown overseas, ready to bring British tourists home on Monday.
The Civil Aviation Authority announced this morning that it will working the government to bring thousands back to the UK.
It said that the repatriation is "hugely complex" and it warned that while it is "doing our best to minimise the impact on passengers", "we do expect there to be significant disruption and delays to travel plans".
Chief executive Peter Fankhauser said the outcome was a "matter of profound regret".
It's triggered the biggest ever peacetime repatriation aimed at bringing more than 150,000 British holidaymakers home.
The tour operator's failure also puts 22,000 jobs at risk worldwide, including 9,000 in the UK.
Good morning
Welcome to Business Live.
Thomas Cook, the UK holiday company, has gone bust, sparking the biggest peacetime repatriation of Britons.
All holidays and flights are cancelled and the Civil Aviation Authority has begun the process of bringing 150,000 holidaymakers back to the UK between now and 6 October.
We will be publishing advice, updates and reaction throughout the day.
Live Reporting
Dearbail Jordan, Simon Read and Becky Morton
All times stated are UK
Thomas Cook Gatwick flights now cancelled
BBC international business correspondent Theo Leggett reports that all Thomas Cook flights out of Gatwick have been cancelled.
Some 16 flights had been due to depart today carrying 3,500 customers.
For help, go to the CAA website here
Fosun 'disappointed' with outcome
Thomas Cook had secured a £900m rescue deal in August led by its largest shareholder Fosun, the Chinese company that owns Club Med.
However, a recent demand from Thomas Cook's banks, which include ROBS and Lloyds, to raise a further £200m in contingency funding in case of any shortfall over the winter holiday period put the deal in doubt.
Fosun said in a statement it was "disappointed" following news of the collapse.
"Fosun confirms that its position remained unchanged throughout the process, but unfortunately other factors have changed."
Thomas Cook boss 'distressed' at company failure
Thomas Cook's chief executive Peter Fankhauser has just spoken.
He says: I would like to apologies to the many thousands of hoteliers and suppliers who have stuck with our business in the good times and bad who now face difficult financial choices of their own.
"It is deeply distressing to me that it has not been possible to save one of the best loved brand in travel."
Thomas Cook - your accommodation
Thomas Cook customers whose holidays are Atol protected do not have to pay any fees to their hotel, according to the Civil Aviation Authority.
However, it admits that it may have to relocate holidaymakers to other accommodation for the duration of their stay.
The CAA advice is here.
All flights and holidays cancelled
Transport Secretary Grant Shapps announced that the government and UK's Civil Aviation Authority has hired dozens of charter planes to fly customers home free of charge.
On Sunday, empty aircraft had already started to be flown overseas, ready to bring British tourists home on Monday.
Thomas Cook - what now for passengers?
The Civil Aviation Authority announced this morning that it will working the government to bring thousands back to the UK.
It said that the repatriation is "hugely complex" and it warned that while it is "doing our best to minimise the impact on passengers", "we do expect there to be significant disruption and delays to travel plans".
Details can be found here on what to do if you are currently abroad.
Thomas Cook collapses
Overnight, last-minute negotiations aimed at saving Thomas Cook failed and the 178-year-old holiday firm has collapsed.
Chief executive Peter Fankhauser said the outcome was a "matter of profound regret".
It's triggered the biggest ever peacetime repatriation aimed at bringing more than 150,000 British holidaymakers home.
The tour operator's failure also puts 22,000 jobs at risk worldwide, including 9,000 in the UK.
Good morning
Welcome to Business Live.
Thomas Cook, the UK holiday company, has gone bust, sparking the biggest peacetime repatriation of Britons.
All holidays and flights are cancelled and the Civil Aviation Authority has begun the process of bringing 150,000 holidaymakers back to the UK between now and 6 October.
We will be publishing advice, updates and reaction throughout the day.