MoneySuperMarket has been fined £80,000 for ignoring customers' marketing email opt-outs.
The Information Commissioner's Office (ICO), which imposed the fine on MoneySuperMarket.com Ltd, said the price comparison website sent millions of emails to customers who had made it clear they did not want to be contacted in that way.
The company sent 7.1 million emails over 10 days updating customers with its terms and conditions. Recipients had previously opted out of direct marketing.
MoneySuperMarket's email included a section entitled "preference centre update" which read: "We hold an email address for you which means we could be sending you personalised news, products and promotions.
"You've told us in the past you prefer not to receive these. If you'd like to reconsider, simply click the following link to start receiving our emails."
The ICO said asking people to consent to future marketing messages when they have already opted out is against the law.
MoneySuperMarket sent the messages between November 30 and December 10, 2016. The ICO's investigation found 6,788,496 were successfully received.
ICO head of enforcement Steve Eckersley said: "Organisations can't get around the law by sending direct marketing dressed up as legitimate updates.
"When people opt out of direct marketing, organisations must stop sending it, no questions asked, until such time as the consumer gives their consent. They don't get a chance to persuade people to change their minds."
Mr Eckersley continued: "Emails sent by companies to consumers under the guise of 'customer service', checking or seeking their consent, is a circumvention of the rules and is unacceptable. We will continue to take action against companies that choose to ignore the rules."
A statement from MoneySuperMarket said: "At MoneySuperMarket, we take the protection of our customers' data and privacy very seriously.
"We apologise unreservedly to the customers affected by this isolated incident and we have put measures in place to ensure it doesn't happen again."