UK holiday company Thomas Cook said customers were delaying booking holidays, uncertain of making plans as they see previously popular destinations such as Turkey as less safe.
Thomas Cook said bookings for the summer period, when it makes all its profit, were 5% lower than last year.
It noted that customers were organising their trips later, continuing a pattern it highlighted in February.
Thomas Cook said prices were holding up, with those for package holidays from Britain up 4%.
But the overall trend was for a later booking pattern, which it blamed on security concerns following a series of attacks in holiday destinations.
An attack on tourists in Istanbul in January caused demand for trips to Turkey to plunge, while last year there were also attacks in Paris, Egypt and Tunisia, making some tourists wary of previously popular destinations.
More customers were opting to holiday in mainland Spain and the Canary and Balearic Islands, as well as the US and Cuba. Fewer wanted to go to Turkey, the company said.
But the company stuck to its forecast for profit to grow, as long as bookings recover.
"This is dependent on seeing a sustained recovery in customer confidence as we progress through the summer season," the company said today.
Analysts expect Thomas Cook's core earnings before interest and tax (EBIT) to come in at about £345m, according to Reuters data, 11% higher than last year.
They said that Turkey was usually a popular destination for German holidaymakers. On a sales basis, Germany is Thomas Cook's second biggest market behind the UK and Ireland.