British Airways (BA) carried 675,000 fewer passengers last month, compared to April 2009.
The airline said its operations were "significantly impacted"by the closure of UK airspace after volcanic activity in Iceland.
BA lost the equivalent of six days of flying, which cost the airline an estimated £100 million.
Global passenger capacity for April was down almost 21%, while traffic fell by 22.4%.
Despite the loss, BA said underlying traffic volumes were on the rise, "although challenges continue to affect the outlook".
www.ba.com
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