But since September 2002, the Nats' monthly figures have generally shown an increase each month compared to the previous year.
Last month, transatlantic departures and arrivals decreased by 3.3%. And while transatlantic services flying non-stop over the UK grew by 2.3%, this compared with an 11.1% increase in June 2008 up from the same period last year.
The news comes after the Official Airline Guide warned last week of rising prices caused by airlines cutting the equivalent of one in every 14 seats this winter.
Nats operations director Ian Hall said: "The aviation industry has always been a barometer of the wider economy so it should not come as any surprise to see a slight slowdown, particularly since the airlines have signalled some cutbacks to services.
"This is the peak summer month and it is important to note that we are still handling record numbers of flights in key parts of our operation."
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