Previously, the company could have used EU-issued carbon credits to settle its 2018 pollution bill.
However, the steel maker has been affected by a European Union decision to suspend UK firms' access to free carbon permits until a Brexit withdrawal deal is ratified.
Sources say there is no danger to British Steel sites or jobs.
The EU's emissions trading system's rules allow industrial poll
The business put its airline up for sale in February and it said last month it would close 21 stores across the country and cut more than 300 jobs.
Chinese investment firm Fosun International could be a bidder for the company's tour operating business, Sky News reported.
Thomas Cook declined to comment.
Fosun International, which runs a joint venture with Thomas Cook in China, did not immediately respond to a request for comment.
Insiders told Sky it could be months before any formal offer is lodged, and there is no guarantee one will be made.
Thomas Cook is the world's oldest tour operator and was founded in Market Harborough in 1841 by businessman Thomas Cook. It started off with organised railway outings for members of the local temperance movement.
It now has annual sales of £9bn, 19 million customers a year and 22,000 staff operating in 16 countries.
In September, Thomas Cook said profits would be hit after the summer heatwave saw many take their holidays in the UK.
It issued a second profit warning in November, when it said winter bookings were also down.
The shop closures will take the number of Thomas Cook outlets down to 566.
It said a consultation with staff and unions had begun.
uters to use carbon credits to pay for the previous year's emissions, or trade them to raise money.
Each free permit gives a firm the right to emit a tonne (1,000kg) of carbon dioxide (CO2), and they can be traded for money.
In a statement the company said: "We are discussing the impact of Brexit on our business with ministers and officials from the Department for Business, Energy and Industrial Strategy and they have been extremely responsive and supportive to date."
The company is in talks with Department for Business about financial help.
The Department for Business, Energy and Industry Strategy told the BBC: "As the business department, we are in regular conversation with a wide range of sectors and companies."
British Steel has until 30 April to comply with EU emission rules.