International Airlines Group (IAG), the parent company of British Airways and Iberia, posted a 6.6% increase in passenger traffic in May compared with the same period in 2011.
Capacity grew 5.9% and load factor increased by 0.5 percentage points to 78.6%.
Group premium traffic for May grew by 1.7% year-on-year, compared to 7.5% growth in non-premium traffic.
British Airways recorded a 10.1% increase in passenger traffic during the month alongside a 9% growth in capacity. Freight traffic remained flat for the period.
Iberia posted a 1.8% decrease in passenger traffic and a 1.8% decline in capacity, with freight traffic falling by 15.5%.
"Underlying market conditions at our London Heathrow hub continue to be firm, particularly in long-haul premium," says the parent company in a statement.
"However, commercial performance at our Madrid hub has deteriorated further due to the ongoing effects of the Spanish and wider Eurozone macroeconomic conditions and the after-effects of prolonged industrial action."