Airport operator BAA said 6.1m passengers travelled through Heathrow in June, a 6.3 per cent increase on the same month last year.
The figures represented the highest number of passengers for the month on record.
But some of the lift compared with the previous year can be attributed to the strike that limited flights during that period in 2010.
BAA chief executive Colin Matthews said: “Heathrow’s position as the UK’s only hub airport enables it to benefit from growth in long-haul routes and emerging economies.
“June’s passenger figures are an early indication of a busy summer, and we will focus on delivering the best possible service standards at our most popular time of year.”
However, cargo traffic at BAA’s airports last month fell 1.5 per cent on last year, and 20 per cent on June 2009 as the economic downturn continued to take its toll on international trade.
BAA also admitted its other London airport, Stansted, is continuing to see declining passenger numbers.
They dropped 3.3 per cent to 1.7 m last month, which is bad timing as the Competition Commission is expected to publish a report next week ordering BAA to sell the airport. BAA has failed to overturn the decision in a series of lawsuits.
Elsewhere, Gatwick airport’s owner Global Infrastructure Partners said it too enjoyed a 5.2 per cent rise in passenger numbers to £3.2m last month.