The company's shares tumbled to a six and a half low as it said discounting and higher fuel and hotel costs would hurt it during the peak summer season.
The 178-year-old business said it was pushed deep into the red for the six months to 31 March by a £1.1bn writedown of United Kingdom package holiday unit MyTravel, which had been revalued "in light of the weak trading environment".
Chief executive Peter Fankhauser said: "There is now little doubt that the Brexit process has led many United Kingdom customers to delay their holiday plans for the summer".
Laith Khalaf, a senior analyst at financial advisory firm Hargreaves Lansdown, said: "Thomas Cook's scaled back the holidays it is offering in response to lower consumer demand, but the competitive environment means that even so, it's having to offer discounts to get customers to part with their cash".
Shareholders reacted to the latest torrent of bad news with another sell-off as shares in the business slumped almost 15% to 19.6p.
"Although it says it has received multiple bids for its airline, in the current climate with an environmental movement suggesting people are trying to make fewer journeys via air, the number of potential investors and the amount they are willing to pay could start to shrink".
Thomas Cook's underlying loss for the half-year was £245m compared with a £170m loss a year ago, while the company has secured £300m of loans ahead of the winter 2019/20 season.
Britain's oldest package holiday firm said it had sold just 57% of its summer 2019 holidays, with tour operator bookings down 12%.
"There's no doubt that we have had a decline in consumer confidence during this whole Brexit phase in the run-up to March, but we have seen no material change to booking patterns in recent weeks since the delay to Brexit was announced", he added.
Fankhauser said the company was assessing bids for its airline unit.
Thomas Cook warned that second-half underlying earnings before interest and tax would be below the same period past year and added it had agreed a £300m bank facility to provide more liquidity for the 2019/20 winter season. The Chinese company Fosun, the largest Thomas Cook shareholder, is among those eyeing up its high street stores and package holiday business.
The debt-laden company has struggled recently, as a fall in demand for package holidays and intense online competition resulted in a string of profit warnings.
Thomas Cook also said it was cutting 150 jobs at its head office in Peterborough as it continues to cut costs. "Tui is the next weakest in the market in terms of its debt".
Thomas Cook faced similar doubts about its survival in 2012, when it was forced to sell hotels and part of its airline to raise money. The company carried out a rights issue in 2013 to shore up its balance sheet.
In a bid to revive its fortunes, Thomas Cook has put together a strategy for profitable growth. Thomas Cook opened 12 more hotels in the first half, taking the total to 200 hotels, including franchised outlets, and is aiming for 250 by 2021. There has been a boom in short city breaks to San Francisco and Las Vegas.