Heathrow’s passenger numbers remained nearly 50% down on pre-pandemic levels last month.
Figures released by the west London airport show it was used by only 2.9 million passengers in February.
That was 15% below the forecast total.
Some 5.4 million passengers travelled through the airport in February 2020, before the coronavirus crisis had a major impact on the aviation industry.
Heathrow said outbound leisure travel is “recovering strongly”, but demand for inbound flights “remains supressed” by the coronavirus testing and quarantine requirements in nearly two-thirds of the markets it serves.
These include higher fuel prices, longer flight times on routes affected by airspace closures, and concerns from US travellers about war in Europe.
The likelihood of new coronavirus variants of concern emerging is also a potential threat to growth.
Together these issues create “huge uncertainty over the passenger forecasts this year”, the airport said.
Heathrow said the strength of outbound leisure bookings reported by airlines indicates its busiest days this summer could see demand return to as high as 85% of pre-pandemic levels.
It warned that check-in areas are “likely to feel even busier” than that because of additional pre-departure checks.
But it said it is “particularly concerned over Border Force’s ability to scale up to meet demand”.
The Civil Aviation Authority (CAA) is expected to make a final decision this year on how much Heathrow can charge airlines for using the airport up to 2027.
The regulator allowed Heathrow to increase charges by more than 50% from the beginning of this year as a temporary measure.
Heathrow chief executive John Holland-Kaye said: “Aviation’s recovery remains overshadowed by war and Covid uncertainty.
“But we need to ensure we are geared up to meet peak potential demand this summer and are relying on the CAA to make a fair financial settlement that incentivises investment to maintain passenger service and encourages airlines and Heathrow to work together to grow passenger numbers.”