British Airways carried 11.1 percent fewer passengers in June compared with the year before, reflecting the impact of walkouts by cabin crew protesting pay and conditions.
BA carried 2.57 million passengers last month, down from the 2.93 million in June 2009, while its load factor -- a measure of how well it fills its planes -- fell 2.1 percentage points to 77.5 percent.
BA said industrial action affected the first nine days of the month. The airline has been hit by a series of strikes organized by the Unite union this year, which have so far cost it 150 million pounds ($232 million).
The union last week postponed a planned ballot on further strikes to allow union members to consider the latest offer from the airline's management.
On a conference call with reporters on Monday, BA's group treasurer George Stinnes said it was positive that cabin crew were being given the chance to vote on the new offer without the union making a recommendation on whether they should accept it.
Unite have said they will give the crew the chance to speak their own mind with respect to the offer that's currently on the table from the company and will offer no recommendation. I certainly think it's positive that staff are being given the chance themselves in a confidential forum to vote, he said. Earlier on Monday, Irish low-cost carrier Ryanair said it carried 15 percent more passengers in June compared with the year before. Its load factor fell by 1 percent to 84 percent.
Shares in BA were down 1.4 percent to 185.8 pence at 1435 GMT, while Ryanair shares were unchanged at 3.68 euros.
(Reporting by Matt Scuffham; Editing by Lorraine Turner and Jon Loades-Carter)