International Airlines Group , formed by the merger of BA and Iberia, posted a strong rise in traffic in December, boosted by continued growth in first and business class passenger numbers.

Traffic, measured in revenue passenger kilometres, rose 12.2 percent versus December 2010, while passenger load factor -- a measure of how well it fills its planes -- was up 0.6 percentage points at 77.6 percent, it said on Thursday.

IAG said its first and business-class travel -- the most profitable part of its passenger business - rose 13.6 percent, while non-premium traffic was up 12 percent.

Earlier on Thursday Ireland's Ryanair , Europe's largest airline, said it carried 4.8 million passengers in December, 5 percent less than a year ago, after it grounded 80 aircraft to avoid flying routes made unprofitable by high fuel prices.

Industry body IATA last month said international passenger traffic growth had slowed in recent months and that it expects airlines to suffer from waning consumer confidence, sluggish international trade and high fuel prices in 2012.

The company said that underlying market conditions were unchanged but that it had seen a significant reduction in Iberia's short haul premium traffic compared to last year, influenced by the strikes during the month.

IAG, which recently agreed to buy Lufthansa's British unit bmi, last October reported a 31 percent fall in third-quarter profit, better than expected and outperforming peers. However, its traffic growth has slowed in recent months.

IAG shares, which have fallen 2.7 percent in the last month, were 2.8 percent down at 145.25 pence by 3:08 p.m., valuing the company at around 2.7 billion pounds.

(Reporting by Rhys Jones; editing by Matt Scuffham)