The security breach occurred late last week, forcing Bitoinica off air as it scrambled to assess the damage.

In a statement, the firm says that the thief made off with 18,547 bitcoins from its deposits. "The overwhelming majority of our bitcoin deposits were not stolen," the statement continues. "The thief stole from us not you. All withdrawal requests will be honoured."

Nonetheless, the exchange operator is warning that the attacker may have made off with customer usernames, e-mails and account history, and urges users to change their passwords and be wary of suspicious e-mails sent to their Bitcoinica accounts.

The breach is the latest in a series of pitfalls to have befallen the peer-to-peer virtual currency. The value of Bitcoin plummeted in June last year after an account at MT. Gox was compromised and a large amount of coins sold off. And earlier this year, e-wallet provider Paxum ceased all business involving the virtual currency after coming under pressure from MasterCard and banks, and major exchange TradeHill shut down.