Epsilon a unit of Alliance data systems announced on Friday that a person outside of their company had hacked into its clients’ customer files.

The company sends out over 40 billion emails every day consisting of ads and offers. Generally these are sent to people who register on the company’s website or who give out their email details while shopping.

Law enforcement agencies are investigating the crime although it is still unclear exactly how many organisations have been exposed.

An Epison spokeswoman, Jessica Simon stated “While we are cooperating with authorities and conducting thorough investigations we are unable to say anything else.”

The incident comes three years after hackers penetrated Heartland Payment Systems, a credit and debit- card processor. In this case notorious hacker Albert Gonzalez led a ring which stole over 40 million payment card numbers. He was subsequently sentenced to 20 years in prison.

Data leaked includes customer names and e-mail contacts. The list of targets includes some of the world’s largest banks such as Capital One, Citibank and JP Morgan Chase.

A JP Morgan spokesperson announced “We are advised by Epsilon that the files accessed did not include any customer financial information, but are actively investigating to confirm this.”

Furthermore Barclays Bank of Delaware posted this to it’s clients “It is possible that you may receive email messages as a result which may ask you for additional information regarding your account. Please be advised that Barclays would never ask you to verify sensitive information such as your full account number, username or password within an email. Therefore any such email should be treated with suspicion.”

The incident has come as a major embarrassment for Epsilon’s banking clients who must contact their clients and advice them to be wary of future marketing communication threats.