The 171-year-old travel group said on Thursday it made an underlying operating loss of £26.5m in its fiscal third quarter compared with a profit of £20.1m in the same period last year.
Net debt at 30 June 30 £1.01bn, versus £902.5m at the same time last year. The firm said that while the outlook remained challenging, its quarterly financial trend was improving and it expects to post a full-year result broadly in line with expectations.
Thomas Cook has been hit hard by tough trading conditions, particularly in Britain where its core customer base of families with young children has been particularly hit by the economic downturn.
It has also been affected by unrest in popular destinations such as Egypt, Tunisia and Morocco.
"My initial focus is to review our businesses, quickly establish priorities and develop a clear plan to reinvigorate Thomas Cook, which I expect to be able to present to you next spring," said chief executive Harriet Green, who joined the company from British electronic parts distributor Premier Farrell in July.
At its half-year results in May, the tour operator posted a pretax loss of £328.3m for the six months to the end of March.