Criminal gangs from eastern Europe are using low-cost airlines to commit crimes in cities across the continent before returning home, according to EU law enforcement agency Europol.
It said the main crimes being carried out are thought to be card-skimming and pickpocketing.
Romanian and Bulgarian gangs are thought to be responsible for 90% of all card-skimming crimes in Europe.
In an interview with the London Times newspaper, Europol Director Rob Wainwright said gangs from Lithuania, Poland and Romania are operating in 20 or more European countries.
He said the criminals fly on low-cost airlines, do a few hits in one city and get back home in time for tea, which makes it very difficult for local police to respond.