In a statement, the company said that a criminal investigation had been launched on Thursday.
The Metropolitan Police, which is investigating, said no-one had been arrested over Wednesday’s attack but enquiries were ongoing.
TalkTalk said there was a chance that some of the following customer data, not all of which was encrypted, had been accessed:
◾Names and addresses
◾Dates of birth
◾Email addresses
◾Telephone numbers
◾TalkTalk account information
◾Credit card and bank details
In the wake of the news, the company’s share price dropped by 11% in the first few hours after the London stock exchange opened at 08:00 BST, their lowest level since August 2013.
Cyber security consultant and former Scotland Yard detective Adrian Culley told BBC Radio 4’s Today programme that a Russian Islamist group had posted online to claim responsibility for the attacks.
He said hackers claiming to be a cyber-jihadi group had posted data which appeared to be TalkTalk customers’ private information – although he stressed their claim was yet to be verified or investigated.
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