Unions representing pilots, cabin crew and travel agency staff reacted with anger over the company's demise, directing much of their criticism at the Government.
Brian Strutton, general secretary of the British Airline Pilots' Association (Balpa), said: "The hopes of all Thomas Cook employees that their airline could survive has been brutally quashed this morning as they wake up to find they have no job.
"While detailed plans to repatriate passengers have been carefully put together and ministers have and will continue to claim the credit for that, the staff have been stabbed in the back without a second's thought.
"Despite continuing to keep Thomas Cook going in recent weeks with dignity and integrity while their own futures were being secretly decided we don't even know if staff will get a pay cheque this month. It is despicable. Thomas Cook pilots and all staff deserve better than this.
"For pilots, Balpa will be supporting our members through the legal complexities of what Thomas Cook liquidation means for them and doing everything we can to help them find alternative jobs in other airlines."
Manuel Cortes, leader of the Transport Salaried Staffs Association, said: "Administration need not have happened - the Government had been given ample opportunity to step in and help
Thomas Cook but has instead chosen ideological dogma over saving thousands of jobs.
"That they would rather hang our members out to dry instead of rescuing Thomas Cook is shameful and wrong-headed.
"There remains the question of repatriating 150,000 British holidaymakers and the cost to the public purse of doing so. You don't have to be a mathematical genius to know it would have been cheaper and more cost effective to save what is a cornerstone of the British high street.
"Our union is still urgently seeking meetings with Government to discuss exactly what happens next and will not give up the fight for jobs, but clearly Tory ministers should hang their heads in shame over their inaction."
Mr Cortes said: "The Business Secretary, Andrea Leadsom, has said she wants an urgent inquiry into the collapse of the firm but the scope of that must include the Government's response, or lack of response.
"It is incredible that with 9,000 jobs on the line she has refused to meet us."
Unite general secretary Len McCluskey said: "News of the needless collapse of Thomas Cook is devastating for the workforce and customers. Thomas Cook Airlines was a profitable and viable operation with a loyal workforce.
"Instead of stepping in and giving Thomas Cook the breathing space it needed to restructure its finances, the Government's 'do nothing' attitude has left workers and customers high and dry while landing taxpayers with a bill of hundreds of millions of pounds."