The slump is the latest indication of the difficulties facing the airline, which in August announced a three-year programme to cut capacity and boost its focus on higher-paying business class passengers.
The disclosure came from BAA, which owns and operates seven British airports, including Heathrow, Gatwick and Stansted. BAA's half-year report, released yesterday, said: "In the context of British Airways' capacity plans and their recent declines in passenger volume [we] would point out that we continue to see loss in their market share picked up by other carriers.... For example, in September, with British Airways' passengers at BAA airports down 4.9 per cent, non-British Airways passenger traffic for BAA grew by 9.5 per cent."
British Airways has been struggling in the face of increased competition on long-haul routes, especially across the Atlantic, and short-haul routes to Europe.
In August, the airline said that it would stop chasing market share, and instead seek to jack up its margins by removing 12 per cent of its capacity over the coming three years.
The company's shares, which have recently fallen to their lowest level in six years, ended 3.75p higher yesterday at 359.25p.
A BA spokesman said: "This [fall in passenger numbers] fits in with our plans to reduce capacity."
BAA's chief executive, Mike Hodgkinson, said that BA was still using all of its slots at Heathrow, about 38 per cent of the total, but was flying smaller aircraft on some of its routes.