The number of passengers British Airways carried in May fell by 14.2% from a year ago, as the impact of cabin crew strikes hit the airline.
The last 14 days of May were affected by industrial activity as members of the Unite union staged walk-outs in a row over pay and travel perks.
The strikes cut passenger capacity by about 6%, BA added.
The financial impact of the strikes continues "at some £7m per day", the airline said.
"The total cost of the strike period can only be assessed at the end of the disruption and will reflect lost bookings and reduced travel volumes offset by some volume driven cost savings," BA said in a statement.
As a second five-day strike comes to an end, another walk-out is expected to begin on Saturday, if no deal is reached between the airline and the union.
Unite has also threatened to ballot for more strikes in July.
May also marked the second month in a row that BA saw a drop in passenger numbers. In April, the airline carried 24.5% fewer passengers than a year earlier after flights were severely disrupted by the volcanic ash cloud.
BA carried a total of 2.37 million passengers in May, down from 2.76 million a year earlier.
Revenue passenger kilometres, a measure of how many full seats flew, were down 11.5%.
Premium traffic fell 6.5%, while non-premium traffic dropped 12.5%.
Aviation analyst and former BA cabin crew manager Jamie Bowden told the BBC that BA would be "relatively pleased" with the figures, "particularly with the premium business traffic".
Mr Bowden added that the Unite union would struggle to get the same level of support for future industrial action.
"If we go back to the February ballot, 80% of cabin crew [who voted] voted for strike action but it's absolutely clear that 80% of cabin crew have not followed through with that," he said.
The airline also repeated its claim that a growing number of strikers were returning to work, and during the next walk-out it plans to fly more than three-quarters of customers who booked flights.
It has also said it expects to operate all flights to South Africa ahead of the World Cup.
These departures will be part of the 80% of long-haul flights it aims to operate from Heathrow next week, up from 70% this week.
It also expects to run 60% of short-haul services from the airport, up from 55% this week.
Flights from Gatwick and London City airports will continue to be unaffected.