Police have launched an investigation into a "significant and sustained cyberattack" on the website of telecommunications firm TalkTalk.
The personal information, credit card and bank details of the phone and broadband provider's four million UK customers may have been accessed during the attack.
However, details of the extent of the attack remain unclear.
The company is working with specialists at the Metropolitan Police Cyber Crime Unit "to establish exactly what happened and the extent of any information accessed," the company said in a statement.
Dido Harding, CEO, said: "TalkTalk constantly updates its systems to make sure they are as secure as possible against the rapidly evolving threat of cyber crime, impacting an increasing number of individuals and organisations.
"We take any threat to the security of our customers’ data extremely seriously and we are taking all the necessary steps to understand what has happened here.
"As a precaution, we are contacting all our customers straight away with information, support and advice around yesterday’s attack."
A Metropolitan Police force spokeswoman confirmed the investigation was ongoing, and said no arrests have been made.
Customers attempting to access the website were greeted with the message: "Sorry we are currently facing technical issues, our engineers are working hard to fix it. We apologise for any inconvenience this may cause."
The incident is the third recent cyber attack to affect TalkTalk.
In August criminals breached the personal data of customers on its mobile site after, after scammers stole thousands of account numbers and names from the company's computers in February.
The thefts were uncovered when TalkTalk investigated a spike in complaints from customers about scam calls between October and December 2014.
Additional reporting by PA
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