The firm could fall into administration in days unless it finds £200m in extra funds needed to secure its future.
The Transport Salaried Staffs Association (TSSA) union has called on Business Secretary Andrea Leadsom to save Thomas Cook "no matter what".
Government sources suggest ministers are reluctant to bail out the company.
TSSA general secretary Manuel Cortes told Mrs Leadsom it was up to the government to save thousands of jobs and to allow Thomas Cook to "flourish".
He said in a letter: "It is incumbent upon the government to act if required and save this iconic cornerstone of the ...
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