Heathrow saw a 4.8% year-on-year decline in passenger numbers in February due to coronavirus.
The airport said the fall to 5.4 million passengers was driven by lower demand on Asian and European routes.
The figure takes into account the extra day last month as 2020 is a leap year.
Heathrow warned that demand has continued to weaken in March.
The airport's chief executive, John Holland-Kaye, said: "The threat of coronavirus is an increasing challenge for the UK and we are working day and night to ensure Britain's front door is open and safe for our people and passengers.
"We will continue to work with the Government to limit the impacts this will have on UK plc."