Bitstamp CEO Nejc Kodrič Twitter/Nejc Kodrič UK-based Bitcoin exchange Bitstamp has been forced to suspend operations after hackers broke into its servers and made away with £3.4 million worth of Bitcoin, TechCrunch reports.

The Bitcoin exchange published a statement on its website which claimed that some of its Bitcoin wallets were compromised on January 4. Hackers didn't steal all of the bitcoin held by Bitstamp, rather, they only managed to gain access to a part of the company's infrastructure.

It's difficult for hackers to make away with all of the Bitcoin held on exchanges like Bitstamp because they store the majority of their Bitcoin in "cold storage." Cold storage uses computers that aren't connected to the internet to store Bitcoin, meaning that hackers can't gain access to the cryptocurrency stored on them.

The CEO of Bitstamp, Nejc Kodrič, took to Twitter to apologise for the disruption to the service:

My sincerest apologies to those who are affected by our service being temporary suspended. Details: http://t.co/SzgFNa1NEM — Nejc Kodrič (@nejc_kodric) January 5, 2015

Bitstamp is one of the largest Bitcoin exchanges in the world, and the leading site in Europe. It was known as the main alternative to Mt. Gox, the Japan-based Bitcoin exchange that shut down in February 2014 after a massive hack resulted in the theft of $473 million in Bitcoin.

This isn't the first time that Bitstamp has had to suspend its service after coming under attack. In February 2014, the Bitcoin exchange came under a Distributed Denial of Service attack (a kind of cyberattack where websites are flooded with traffic to knock them offline). Hackers then sent a ransom note to the exchange, asking for money to stop the barrage of fake web traffic.