Supermarket giant Tesco has deactivated more than 2,000 of its customers' online accounts after their personal details were published on text sharing site Pastebin.
The leaked details included the email addresses, plain text passwords and Tesco Clubcard point balances of 2,239 of the company's customers.
According to a report by the BBC, the data may have been pieced together by hackers using information lifted from other sites and cyber attacks.
It is thought the gleaned email addresses and passwords were then systematically used by hackers to try and access Tesco.com accounts.
In a small number of cases, the hackers are said to have stolen Clubcard points from customers too, which Tesco has agreed to reimburse.
In a statement on the Tesco Facebook page, the company said it was investigating the breach.
"We take the security of our customers' data extremely seriously and are urgently investigating these claims," a company spokesperson wrote.
"We are committed to ensuring that nobody misses out as a result of this. We will issue replacement vouchers to the very small number who are affected."
Trey Ford, global security strategist at security vendor Rapid 7, said the case highlights the perils of using the same login details across multiple online accounts.
"So far the information available indicates the impact of this has been relatively limited stolen vouchers but if attackers have tried this on Tesco.com, the chances are they are also trying it on other sites too and so we may see additional fallout," said Ford.
"This is [a lesson] in consumer behaviour people continue to reuse passwords and other credentials across multiple sites, making it easy for attackers to compromise them. It's essential to learn the lesson from this incident before the cost becomes greater," he added.
