Figures released by the west London airport show it was used by 2.9 million passengers last month.
In February 2020, just before the pandemic caused people to stay at home, it was used by 5.4 million.
An airport spokesperson said outbound leisure travel was “recovering strongly” but demand for inbound flights remained supressed.
Coronavirus testing and quarantine requirements still apply in nearly two-thirds of the markets it serves.
The spokesperson added that the industry faced “headwinds” resulting from Russia’s invasion of Ukraine such as 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 was also a potential threat to growth, the spokesperson said.
The latest figures are 15% below what was forecast.
However, the spokesperson said the strength of outbound leisure bookings reported by airlines indicated its busiest days this summer could see demand return to 85% of pre-pandemic levels.