BAA, the airport operator, has blamed the "Olympics effect" for a fall in passenger numbers at Heathrow and Stansted in August.
The company said traffic through Heathrow in July and August was "lower than expected and the shortfall is not expected to be recovered later in the year".
BAA, owned by the Spanish group Ferrovial, reported that visitors to Heathrow fell by 1.9 per cent to 6.5 million in August, while they tumbled by 5.2 per cent at Stansted. Passenger numbers fell by a more "pronounced" 4.6 per cent at Heathrow in the first two weeks of last month, against a 0.3 per cent rise over the rest of August at Europe's busiest airport.
A BAA spokesman said: "This suggests a continuation of the 'Olympics effect' reported in July, with UK passengers staying at home, as well as non-Olympic visitors from overseas choosing to defer their journeys."
Heathrow's European scheduled traffic suffered, with only Norway and Denmark growing by 4 per cent and 1.2 per cent, respectively.
Overall, BAA, which also operates Southampton, Aberdeen and Glasgow airports, said passenger traffic fell by 2 per cent to 9.5 million across its five sites last month. The performance of these three airports differed widely. While Southampton suffered a 6.4 per cent fall, Aberdeen's passenger traffic rose by 9.2 per cent and Glasgow was up by 3.1 per cent.