TalkTalk, a phone and broadband provider based in the UK, has announced a “significant and sustained Cyber-attack” on its website. The company said all of its customers could potentially be affected by the security breach, even though it was too early to know the type of data that has been compromised.

TalkTalk has over four million customers, and the company’s share price fell by 10% at the London Stock Exchange in the hours following news of the attack.

A Russian Islamic group allegedly claimed responsibility for the attack through online postings, according to former Scotland Yard detective, Adrian Culley.

Some of the customer data which could have been accessed during the attack include names, addresses, credit card and bank information, email addresses and dates of birth. Not all of this information was encrypted.

This is the second cybersecurity incident, following an intrusion at the later part of 2014, when some customer data was stolen. TalkTalk has contacted its customers, saying that it has “taken all necessary measures to secure” its website.