The Government is being urged to step in to save Thomas Cook from going out of business amid a possible collapse that could leave thousands of holidaymakers stranded.
The travel company could fall into administration within days unless it finds £200 million in extra funds.
The money is on top of a £900 million package it had already agreed to see it through the winter months when it needs to pay hotels for their summer services.
A source said the company was in talks with the Government and a number of potential investors about bridging the funding gap.
It will hold a board meeting on Saturday and Sunday to evaluate its position.
"We have not given up," the source told Reuters news agency.
The Transport Salaried Staffs Association, which represents workers at the company, said the Government should be ready to assist with "real financial support".
General Secretary Manuel Cortes called for an urgent meeting with Business Secretary Andrea Leadsom.
He said in a letter: "It is incumbent upon the Government to act if required and save this iconic cornerstone of the British high street and the thousands of jobs that go with it.
"Thomas Cook can be a highly successful business and must be given every opportunity to flourish. I urge you to stand ready to assist Thomas Cook with real financial support.
"The company must be rescued no matter what. No British Government in its right mind would countenance the loss of so many jobs and the prospect of just one major travel operator - TUI - controlling the mass market."
It is understood that Thomas Cook has approached the Government in an attempt to plug a gap in its funding.
Should Thomas Cook fall into administration it would spark the largest peacetime repatriation in UK history with 160,000 currently enjoying Thomas Cook holidays in destinations including California, the Caribbean and Corfu.
In total, some 600,000 holidaymakers from markets including Germany and Scandinavia could be stranded.
The company's social media channels are full of customers asking if they will be able to get home.
The British government and airline regulator have already drawn up plans in case they need to step in to bring customers home. But the fallout from any collapse would go far beyond the interrupted holidays of its customers.
Founded in 1841, Thomas Cook runs hotels and resorts, airlines, cruises and hundreds of high street travel agent stores. With 21,000 staff, it operates in 16 countries and serves 19 million customers a year.
The company's demise could affect the economies of its big holiday markets Spain, Greece and Turkey, its shareholders and lending banks, and its many British high street stores.
Its largest shareholder is China's Fosun, which was due to take a central role in the rescue package.
Additional reporting by agencies