Police are investigating after a "significant and sustained cyber-attack" on the TalkTalk website, the UK company has confirmed.
The phone and broadband provider, which has over 4 million customers in the UK, said credit card and bank details could have been accessed.
A TalkTalk spokesperson said: "A criminal investigation was launched by the Metropolitan Police Cyber Crime Unit following a significant and sustained cyber attack on our website yesterday.
"That investigation is ongoing, but unfortunately there is a chance that some of the following data has been compromised: names, addresses, date of birth, phone numbers, email addresses, TalkTalk account information, credit card details and/or bank details.
"We are continuing to work with leading cyber crime specialists and the Metropolitan Police to establish exactly what happened and the extent of any information accessed."
A Metropolitan Police Cyber Crime Unit spokesperson said: "We are investigating an allegation of data theft.
"The theft was reported to the Met on Wednesday, 21 October. There have been no arrests and enquiries are ongoing."
The company's chief executive, Dido Harding, 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."
TalkTalk was informing its customers immediately about the attack as a precaution, she added.
Ms Harding said the website was now secure again and the company had also contacted the major banks asking them to monitor for any suspicious activity on customers' accounts.
She said that every customer would also be getting a year's free credit monitoring.
A Police spokesman confirmed they were investigating an allegation of data theft.
He said: “There have been no arrests and enquiries are ongoing."
