The once-mighty Mt Gox exchange reportedly lost them in a cyber attack, with an apparent internal document suggesting more than 740,000 Bitcoins are missing, accounting for 6% of Bitcoins in circulation and worth roughly $375m, Sky News reports.

The Mt Gox website is currently offline with trading suspended and withdrawals have been frozen.

The shutdown comes days after Mt Gox chief executive Mark Karpelès stepped down from the board of the Bitcoin Foundation – an advocate for the virtual currency.

The firm’s Tokyo office is said by witnesses to be largely bare while a handful of angry investors are camped outside demanding news of their money and questioning whether the business remains solvent, Sky News says.

One of them, Bitcoin trader Kolin Burgess, said he had picketed the building since February 14 after flying in from London, hoping to get back $320,000 he had tied up in Bitcoins with Mt Gox.

“I may have lost all of my money. It hasn’t shaken my trust in Bitcoin, but it has shaken my trust in Bitcoin exchanges.”

The fate of his holding is unclear as Bitcoins are traded in an unregulated, decentralised marketplace.