Heathrow aiport saw a record 6.9m passengers in July.
The month also included the airport's busiest day in its history on Sunday 31 July, airport owner BAA said.
Passenger numbers were up 2.5 per cent on the same period a year ago.
However, Stansted numbers fell 7.2 per cent.
BAA's three Scottish airports - Glasgow, Edinburgh and Aberdeen - all saw increases in passenger numbers.
For BAA as a whole, passenger numbers rose 0.8 per cent to 11m.