Airport operator BAA said it lost nearly 23% of its passengers last month, compared to April 2009, as a result of the volcanic ash cloud.
In April, BAA's UK airports, which include Heathrow and Stansted, handled 6.9 million passengers, 1,900,000 fewer than expected.
Heathrow alone lost 1,180,000 passengers in April, the largest drop for BAA airports.
The second largest fall was seen at Stansted, which lost 330,000 passengers, representing 21% of its expected traffic.
www.baa.com