BBC News report just in:
Police are investigating after a "significant and sustained cyber-attack" on the TalkTalk website, the UK company has said.
Police are investigating after a "significant and sustained cyber-attack" on the TalkTalk website, the UK company has said.
The phone and broadband provider, which has more than four million customers in the UK, said credit card and bank details could have been accessed.
The Metropolitan Police Cyber Crime Unit is investigating the attack, which happened on Wednesday.
It is the second time the company has been targeted by hackers this year.
TalkTalk said in a statement that a criminal investigation had been launched on Thursday, adding there was a chance that customers' names, addresses, dates of birth, email addresses and bank details had been accessed.
A spokeswoman for the Met said: "There have been no arrests and enquiries are ongoing."
Dido Harding, chief executive of the TalkTalk group, told BBC News: "We brought down all our websites [on Wednesday] lunchtime and have spent the last 24 hours investigating with the Met Police.
"It's too early to know exactly what data has been attacked and what has been stolen... Potentially it could affect all of our customers, which is why we are contacting them all by email and we will also write to them as well."
It is expected to take some time to contact everyone and a number of customers have taken to social media to express their frustration that they are yet to hear anything.
TalkTalk urged customers to keep an eye on their accounts over the next few months and report any unusual activity to their bank and Action Fraud on 0300 123 2040.
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.