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 inceased spam and phishing attacks.