The problem was caused by one of the company's suppliers mistakenly sending an email, which if it was replied to was sent to all the other people in the group.
Virgin Media insisted only a "small proportion" of its customers were affected, but the list of people in the group was more than 130,000.
Angry customers complained the data breach meant their email address was seen by thousands.
Liz French said: "Not only did it inconvenience many thousands of people and compromise their online security, it may well have cost small businesses both money and customers. At no point did Virgin get in contact with those affected or issue an apology."
A Virgin Media spokeswoman apoligised and confirmed the email address had been disabled.
She advised people not to respond to the email.