ECB concerned virtual currencies may not be sufficiently guaranteed

Bitcoin, the original and still the biggest cryptocurrency, has rallied nearly 75 percent in barely a month.

"We are concerned that this is private currency while we still think currencies have something to do with trust," Mersch told a conference in Luxembourg.

"The public at large has more trust in public currency rather than in private currency where no one knows who is really the issuer and to what extent it is guaranteed," he added.

(Reporting by Robert-Jan Bartunek; Editing by Gareth Jones)