TalkTalk has confirmed that police are investigating a major cyber-attack on its website.
The provider admitted banking details and personal information may have been accessed, warning that all of its four million UK customers may have potentially been affected.
Find out more about the breach and what to do to stay safe with our guide: TalkTalk hack FAQ
The company did stress, however, that it was too early to confirm exactly what data had been stolen.
TalkTalk has confirmed that a criminal investigation has been launched, which BBC News claims is likely to be a distributed denial of service attack, which is when a website is crashed by being overloaded with traffic, serving as a distraction while criminals help themselves to pieces of data.
In a statement, the company said: "We would like to reassure you that we take any threat to the security of our customers’ data very seriously.
"We constantly review and update our systems to make sure they are as secure as possible and we’re taking all the necessary steps to understand this incident and to protect as best we can against similar attacks in future.
"Unfortunately cybercriminals are becoming increasingly sophisticated and attacks against companies which do business online are becoming more frequent."
It added that it had "taken all necessary measures" to secure the website and that the major banks, as well as the Information Commissioner's Office had been contacted.
The Metropolitan Police says enquiries are still ongoing, with TalkTalk advising customers to keep an eye on their accounts over the next few months and to report any unusual activity to their bank or to Action Fraud, the UK's national fraud and internet crime reporting centre.
The provider has also urged customers to check their credit report and ensure they take all steps to check the true identity of anyone getting in contact to ask for personal data or passwords.