British Gas says it has contacted more than 2,000 customers after their login information was leaked online.
The energy company said no credit card information had been revealed, but warned that login information could have been used to names, addresses and previous bills. British Gas said it believes that its systems were not hacked.
In an email to affected customers, British Gas said: "I can assure you there has been no breach of our secure data storage systems, so none of your payment data, such as bank account or credit card details, have been at risk.
"As you'd expect, we encrypt and store this information securely.
"From our investigations, we are confident that the information which appeared online did not come from British Gas."
Marks and Spencer
A similar scenario was reported by Marks & Spencer on Wednesday.
The retailer took down its website for two hours on Tuesday after other customers' details and previous orders were revealed when logging into an account. Marks & Spencer also said it was likely that this was not due to a cyber attack.