Heathrow handled a record 81.5 million passengers in the 12 months to May.
The milestone was reached as the airport welcomed 7.2 million passengers last month, a rise of 6.7% and the first time May traffic has exceeded seven million.
This summer is expected to see more records broken with an estimated 30 million passengers using the London hub.
The European Union was the top destination accounting for almost 2.5 million passengers last month, followed by North America at almost two million.
Heathrow chief executive Thomas Woldbye said: “We have a winning team at Heathrow which has proven that we have put Covid firmly behind us.
“Thanks to their extraordinary efforts we are now giving record numbers of people the chance to connect smoothly with the world.
“Supporting 81 million journeys doesn’t just help families to make wonderful holiday memories, importantly it is about the vital trade and business links a hub like Heathrow creates for the UK’s economy.”
Meanwhile, Stansted pointed to the May half-term holiday week for boosting the month’s passenger levels by 6.3% year-on-year to a record-breaking 2.7 million – surpassed the previous busiest May in pre-Covid 2019 by 143,000 passengers.
The 12 month rolling total to the end of May stood at 28.8 million passengers, an increase of 9.8% on the previous year.
The top destinations during the month were Dublin, Istanbul and Edinburgh, with the most popular countries being Spain, Italy and Germany.
May also saw the return of British Airways to Stansted, with new services to Ibiza, Nice and Florence.
The airport is set to welcome thousands of football fans who will be travelling to the Euros in Germany over the next month.
“Stansted serves 10 German destinations so all the tournament cities will be easily accessible for fans of both England and Scotland, plus many other European nationals based in the UK who will be heading off to follow their teams,” a spokesperson said.