Inbound passengers to the UK dipped 2% year on year in May 2017.
The decline compares with a 13% rise in January to April compared with the same period in 2016.
The number of visitors from Europe fell by 10% in May, while visitors from North America were up 35%.
The British Hospitality Association, which released the figures in its latest travel monitor, believes the fall could be down to the terror attacks in London in March and in Manchester on May 22.
"The differentiation in inbound tourism growth from the two continents could show long-haul travel holding up better in the wake of the terrorist incidents due to the longer booker period to travel," it said.
Despite the 2% fall in visitor numbers, overall UK spend by overseas residents grew 5%, likely due to growth of inbound tourism from North America.
BHA chief executive Ufi Ibrahim said: "Our analysis has shown that while inbound tourism has grown significantly, there may be a short-term drop due to the tragic terrorist incidents earlier this year."
