Some 25 million bags went missing in the world's airports in 2009, costing the airline industry some 2.5 billion dollars, data released Thursday by air transport electronics group SITA.

Just over half of these bags, 52 percent, were misplaced during aircraft transfers, while another 16 percent did not arrive at the destination on time as they were not loaded on planes, luggage tracking data showed.

Other problems included ticketing errors, mishandling at the arrival point, and tagging problems, said SITA.

It was citing data from the World Tracer luggage tracking database, which is co-sponsored by the company and airline industry group IATA.

The vast majority of the missing luuggage was traced and returned within 48 hours.

But 3.4 percent of the total - 850,000 items - either remained unclaimed or were stolen, added SITA, which makes luggage handling systems.

But despite the millions of bags that went missing last year, SITA said the figures were an improvement of 23.8 percent on the previous year, when 32.8 million bags failed to arrive on time.

"Some of the decline can be attributed to fewer passengers travelling last year, but the 2.9-percent decline in passenger numbers is still far smaller than the 23.8 percent decline in mishandled baggage," it said.

Improvements in baggage handling systems and passengers checking in fewer bags to avoid extra fees had also helped improve the figures, it added.