BAA airports handled more than 10 million passengers in September, up 1% on the same month last year.
However, the airports operator warned capacity constraints are threatening connections to emerging markets around the world.
BAA chief executive Colin Matthews said: “Existing routes are performing well, but capacity constraints mean the UK is lagging behind in its connections to emerging market destinations – Paris and Frankfurt already boast 1,000 more flights a year to China’s three largest cities.”
Heathrow saw passenger numbers grow by 1.4% to 6.3 million, a record for September, with aircraft movements and the number of seats filled per aircraft also at record levels.
North Atlantic routes continued to be the fastest-growing market from the London hub with a 3.5% rise in passengers, buoyed by a 5.4% increase in flights.
But a 10.5% drop in domestic services contributed to a 14.5% decline in passenger numbers on UK domestic routes.
Both European scheduled and other long-haul markets to and from Heathrow recorded passenger increases of 2.5%.
Edinburgh airport saw 3% growth – its busiest September and seventh consecutive busiest month on record – while passenger numbers grew by 3.2% at Glasgow.
Aberdeen continued its recent strong run of results with an increase in passengers of 11.3% on September 2010.
Southampton recorded an increase in passenger numbers for the third consecutive month, with a rise of 0.7%.
Planned cuts in services at Stansted resulted in a 4% drop in passengers.
Matthews said: “Passenger numbers rose in September, with stronger growth across the group than in July and August.
“Slightly softer Heathrow traffic in the latter part of September, vigilance in the near term from individual carriers and problems facing the European economy make us cautious about predicting traffic growth trends in the coming months.”