The high street retailer has had to inform customers that their details might have been compromised and as a result they might be victims of more spam and web scams.
The ripples from the hack into Epsilon continues to roll out across both sides of the atlantic with household names having to reveal they have been involved.
M&S has emailed customers to warn them of the situation following similar messages already put out by Ritz-Carlton Hotels, Target and some large US banks.
Robert Chapman, CEO of Firebrand Training, said that the breach was stacking up the bad press for some major brands.:
"These organisations must ensure that their partners are educating staff on the dangers of cyber security, and updating their security systems sufficiently. They should also have business continuity plans in place for when it all goes wrong," he said.
"Although the initial damage is to the organisation's reputation, the worst is to follow. There's a potential bounty available to the hackers - information is power, and they've just gained millions of opportunities to steal from an individual," he added.