The value of bitcoin fell sharply last night after a major exchange halted trading, saying it had uncovered a security breach.
Bitcoin fell as low as $483.10, after Hong Kong-based Bitfinex said some of its users had had their bitcoins stolen.
In a statement it said the theft had been reported to the police.
"As we account for individualised customer losses, we may need to settle open margin positions, associated financing, and/or collateral affected by the breach," it added.
"While we are halting all operations at this time, we can confirm that the breach was limited to bitcoin wallets; the other digital tokens traded on Bitfinex are unaffected."
The cryptocurrency had recovered some of its losses this morning, trading at $545, about 1.4 per cent down on last night's closing price.
The price of bitcoin jumped in the run-up to the EU referendum, at one point rising close to $770. But it has dipped over the last few days, dipping below $600 for the first time since the beginning of June on Monday.
However, prices have remained reasonably steady, despite the so-called halvening.