The FTSE 100-listed flag carrier said it flew passenger 8.3mln passengers last month, up from 7.7mln in December 2017, an 8.1% year-on-year increase.
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However, its load factor - the number of passengers as a proportion of seats available – slipped to 80.4% in December, compared to 80.8% a year earlier.
For 2018 in total, IAG - which also operates the Iberia, Vueling and Aer Lingus airlines - reported passenger numbers were up 7.7% to 112.9mln from 104.8mln in 2017.
Its overall load factor for 2018 improved to 83.3%, up from 82.6% the year before.
Ryanair reported passenger numbers for the month of 10.3mln, up 12% year-on-year (YOY) while its load factor was at 95%.
Meanwhile, Wizz Air reported passenger numbers for the month had risen 18.3% YOY to around 2.6mln while the load factor increased 1.3 percentage points (ppts) to 88.8%.