Thomson and First Choice owner TUI said summer holiday bookings to Turkey had slumped by 40% after recent terrorist attacks.
The travel giant said holidaymakers were staying away from the country and turning instead to resorts in Spain and the Canary Islands.
But TUI said it had got off to a good start to its financial year and kept its guidance for a 10% rise in annual underlying earnings despite the impact from attacks in a raft of popular tourist destinations in recent months.
Terrorist attacks in Turkey, Paris, Egypt and Tunisia have hit the travel sector, with the outbreak of the Zika virus adding to the woes.
Friedrich Joussen, chief executive of TUI, said: "It is evident that there has been a significant shift in demand away from Turkey, with summer 2016 bookings to that destination currently down around 40%."
But he added: "We have delivered a good underlying performance in the first quarter in spite of the backdrop of geopolitical turbulence in some of our destinations."
TUI said demand for holidays in Turkey fell particularly in its German business after 10 German tourists were killed in a suicide bombing in Istanbul last month.
A total of 14% of TUI customers travelled to Turkey last summer.
Shares in TUI fell 3% after the update.
The group saw a 7.2% improvement in seasonal underlying losses in its first quarter to 97.3 million euro (£75.3 million).
It is a key time of year for holiday firms, when many customers book their summer holidays.
Turkey has also been impacted by a drop in demand amid general security fears, as it borders with Syria, which is gripped in a civil war between its government and rebel groups, including Islamic State.
TUI's results for the year to the end of last September showed the impact from the beach massacre in Tunisia in June, as well as the move to cancel all flights to Sharm el-Sheikh in Egypt following the terrorist bombing of a Russian airliner.
TUI has 13 hotels in Sharm el-Sheikh, with the resort accounting for half of its business in Egypt.
The group took a 52 million euro (£37.6 million) hit from the Tunisia attack and former joint chief executive Peter Long said in December the atrocity had been ''the most tragic event that I've ever had to deal with in my 30 years in the industry''.
But strong trading and currency exchange rates helped it offset the cost to post a 23% leap in underlying earnings to 1.07 billion euro (£775 million) for the year to the end of September.
In its latest update, TUI said it had seen a 16% rise in demand from UK holidaymakers for long-haul bookings over the winter following the recent attacks across Europe, with destinations such as Mexico, the Dominican Republic and Jamaica becoming more popular.
But there is also mounting evidence that the Zika virus could be having a damaging impact on customer appetite for holidaying in the Caribbean, after Jamaica announced it had been infected by the virus.
Overall winter UK bookings are up 3%, while selling prices are 1% higher, according to TUI.
Summer bookings in its UK business are 9% higher.
Rival Thomas Cook will reveal how it has fared in its trading update on Thursday.