Vodafone says hackers have accessed the accounts of 1,827 customers, just a week after a separate hacking attack at TalkTalk.
The hack could potentially give criminals customers' names, mobile numbers, bank sort codes and the last four digits of their bank accounts.
The company said an attempt had been made to access some of its customers' account details between midnight Wednesday and midday on Thursday.
An investigation has been launched by the company and the National Crime Agency and communications watchdogs were told on Friday.
A Vodafone spokesman confirmed 1,827 customers had their accounts accessed but added that the information obtained by the hackers "cannot be used directly to access customers' bank accounts".
Hackers Access Nearly 2,000 Vodafone Accounts