The affected individuals are customers of companies that Epsilon provides email marketing services to, including retailers Target, Kroger and Walgreens, hotel chains Marriot and Hilton and the DVR firm Tivo. Epsilon says 2% of its 2,200-plus clients have been affected.

In a statement, the company said: “On 30 March, an incident was detected where a subset of Epsilon clients’ customer data were exposed by an unauthorised entry into Epsilon’s email system. The information that was obtained was limited to email addresses and/or customer names only.

“A rigorous assessment determined that no other personal identifiable information associated with those names was at risk. A full investigation is currently underway.”