Marks and Spencer customers have been warned to expect an increase in spam e-mail after hackers stole their details.
The company has contacted users of its online service to warn them about the data breach, which was part of a wider attack on marketing firm Epsilon.
A number of American companies also had their mailing lists compromised, including the hotel chains Marriott and Hilton, as well as several banks.
Marks and Spencer said that customers' financial details were safe.
"We have been informed by Epsilon, a company we use to send e-mails to our customers, that some M&S customer e-mail addresses have been accessed without authorisation," the retailer said in an e-mail sent on Tuesday evening.
"We wanted to bring this to your attention as it is possible that you may receive spam e-mail messages as a result.
"No other personal information, such as your account details, has been accessed or is at risk."
Epsilon admitted on 1 April that an "unauthorised entry" to their systems had taken place on 30 March.
According to its website, Epsilon serves 2,500 clients and sends over 40 billion e-mails annually.
They clarified on Monday that the breach affected "2% of total clients" - among them many big banks and retailers.
"A rigorous assessment determined that no other personal identifiable information associated with those names was at risk. A full investigation is currently underway," the US-based company said.
Marks and Spencer's statement told customers that it will "continue to work diligently to protect your personal information".
Last week, a similar security blunder by marketing firm Silverpop led to customers from entertainment retailer Play.com being put at risk of increased spam and phishing attacks.