Vodafone UK said hackers accessed the accounts of 1 827 of its customers last week, the second cyber attack on a British telecoms company in October.
The attackers had potentially gained access to the victims' bank sort codes and the last four numbers of their bank accounts, along with their names and mobile telephone numbers, a Vodafone spokesman said.
"This incident was driven by criminals using e-mail addresses and passwords acquired from an unknown source external to Vodafone," he added in a statement.
Only a handful of those affected in the attack had seen any attempts to use their data for fraudulent activity on their Vodafone accounts.
"No credit or debit card numbers or details were obtained. However, this information does leave these 1 827 customers open to fraud and might also leave them open to phishing attempts," the spokesman said.
The company was contacting all those involved and other customers need not be concerned, he said.
Teen threat
Meanwhile, broadband, TV, mobile and fixed-line service provider TalkTalk was hacked earlier last month, potentially putting the private details of its four million customers into the hands of criminals.
It said later that under 21 000 unique bank account numbers and sort codes had been accessed.
Police have made three arrests in connection with the TalkTalk cyber attack.
A 20-year-old man from Staffordshire, central England, was being held in custody and his address was being searched, London police said in a statement.
A 16-year-old boy was arrested in west London and later released on bail. Police also detained a 15-year-old boy in Northern Ireland who was released on bail.
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