Tech Bureau, which operates the  ironically-named exchange, discovered the loss on Monday and then reported it to the authorities the following day.

The thieves made off with at least $40m worth of customers' assets, made up of 5,966 bitcoins and an unknown amount of Bitcoin Cash and Monacoin.

Hackers were reportedly able to access the assets via a 'hot wallet' which is connected to the internet, as oppsed to an offline 'cold wallet' which is preferred by other exchanges as a safer way to hold customers' assets. 

The theft comes at a time when international regulators are questioning the standard and governance and security in the crypto exchange market and looking to impose more onerous rules on many of its operators. 

It is also another black mark for Japan's Financial Services Agency. Hackers stole $500m from Coincheck, another Japanese exchange, earlier this year. The regulator has been accused of authorising several new exchanges without ensuring proper security was in place and then failing to act after security breaches.

More embarassingly for the FSA, Zaif was one of seven exchanges sanctioned for its security measures in the wake of the Coincheck hack. 

Tech Bureau has issued a fulsome apology. "We humbly apologise for betraying the trust of all our customers who have entrusted us with their precious assets,” it said.

It has also said that it is currently looking to sell a majority stake in the exchange from Japanese company Fisco in return for $45m in cash which will be used to compensate customers.