The details of some British Gas customers have appeared online, in the third problem to affect a major UK company in a week.
The firm insisted its systems are secure, after sending out an email to customers informing them of the incident.
A small number of customer details briefly appeared online but our systems are secure.
Details will be sent to the Information Commissioner’s Office following the leak, it said.
In reply to a customer’s query about the incident British Gas Help posted on Twitter: “A small number of customer details briefly appeared online but our systems are secure.”
The email addresses and account passwords for around 2,200 customers appeared online, the BBC reported.
The email from British Gas claimed the information had not come from the company, and said no payment data would have been at risk, the broadcaster said.
On Tuesday evening Marks & Spencer had to suspend its website for two hours after customers were able to see other people’s details when they logged in to their accounts.
The company said no one’s details were compromised by the “internal technical problem”.
Last week phone and broadband provider TalkTalk was targeted in a cyber attack in which it said bank account numbers and sort codes, like those printed on a cheque, may have been accessed.
A joint operation between the Met’s cyber crime unit, the PSNI’s cyber crime centre and the National Crime Agency is ongoing into the incident.
It is not known how many of the telecoms giant’s four million UK customers may have been affected by the attack.