Hackers may have accessed the bank details of nearly 2,000 Vodafone customers, the company has said.
The mobile phone provider said 1,827 accounts were accessed, potentially providing criminals with customers' names, mobile numbers, bank sort codes and the last four digits of their bank accounts.
A Vodafone spokesman said: "This incident was driven by criminals using email addresses and passwords acquired from an unknown source external to Vodafone.
"Vodafone's systems were not compromised or breached in any way."
The breach comes just over a week since telecoms giant TalkTalk said it had been subjected to a "significant and sustained" attack on its website, which prompted fears that millions of customers may had their bank details stolen.
Vodafone said an attempt had been made to access some of its customers' account details between midnight on Wednesday and midday on Thursday.
An investigation was launched by the company before the National Crime Agency (NCA), Ofcom and the Information Commissioner's Office were informed on Friday evening.
A Vodafone spokesman said: "Whilst our security protocols were fundamentally effective, we know that 1,827 customers have had their accounts accessed, potentially giving the criminals involved the customer's name, their mobile telephone number, their bank sort code, the last 4 digits of their bank account.
"Our investigation and mitigating actions have meant that only a handful of customers have been subject to any attempts to use this data for fraudulent activity on their Vodafone accounts."
Vodafone said no credit or debit card details were accessed and the information obtained by the criminals "cannot be used directly to access customers' bank accounts".
"However, this information does leave these 1,827 customers open to fraud and might also leave them open to phishing attempts," the spokesman added.
"These customers' accounts have been blocked and affected customers are being contacted directly to assist them with changing their account details."
Vodafone said it had contacted the banks to alert them and also intends to contact all affected customers, adding that non-affected customers need not be concerned.