The number of flights handled by air traffic control organisation Nats was ten percent lower in January 2009 compared to January 2008.
A total of 167,322 flights were handled in UK airspace in January – a fall from January 2008’s figure of 185,885.
Forecasters had expected a decrease because this January had an extra weekend day, with Saturdays and Sundays seeing fewer flights than weekdays.
Commenting on the figure, a Nats statement said: "All centres and airports where Nats provides the service saw a reduction in traffic this January, with the domestic market down by 12 percent, transatlantic arrivals and departures down by 9.5 percent and ‘other’ arrivals and departures down by 10.1 percent."
Short-haul flights between Ireland and France were also up, recording an increase of 12.4 percent.
Nats announced last month that it had handled 2.4 million flights in 2008, a 1.5 percent decrease from 2007’s figure.