FRANKFURT (Reuters) - Europe's debt crisis may be threatening to unravel the euro but criminals increasingly see the single currency as worth counterfeiting.

The number of fake euro banknotes found in the second half of last year rose by 4.7 percent to 310,000 bills from the prior six months, the European Central Bank said Monday, though it hastened to add that the vast majority of euros were real.

"When compared with the number of genuine euro banknotes in circulation (on average 14.4 billion during the second half of 2011), the proportion of counterfeits remains very low," the ECB said in a statement.

The 20 and 50-euros banknotes remain counterfeiters' favorites, accounting for four out of five fake bills.

(Reporting by Sakari Suoninen; Editing by Toby Chopra)