Virgin Media has apologised after thousands of customers had their inboxes bombarded with unwanted emails due to an error.
The problem was caused by one of the company's suppliers mistakenly sending out 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.
Customers reacted angrily to the data breach, complaining it meant that their email address was visible for thousands to see.
Liz French said: "As a leading technology provider, I find it indefensible that Virgin can allow an error on this scale to occur.
"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. I hope the Data Commissioner looks into this very seriously."
A Virgin Media spokeswoman confirmed the email address had been disabled.
She said: "A small proportion of our customers have received an email from one of our suppliers which, if they reply-all, it is sent to a wider group. We're investigating exactly what has happened and, in the meantime, advise people not to respond to this email. We apologise for the inconvenience caused."