Payment details, names and addresses have been accessed or stolen in the attack on Debenhams Flowers, the retailer has confirmed.
A cyber attack has compromised the personal data of up to 26,000 Debenhams customers.
The breach, which is understood to have been malware-based, targeted the online portal for the retailer's florist arm, Debenhams Flowers.
Debenhams has stressed that the site is operated by Ecomnova, a third-party supplier, and that customers of other services have not been affected.
Ecomnova also operates Debenhams' websites for hampers, personalised gifts and wines. While all four sites have been suspended, the retailer has not announced whether the others were also breached.
Debenhams confirmed to Sky News that customer payment details, names and addresses were accessed or stolen during the attack.
In a statement the company stressed that it was only the Ecomnova-run site that had been compromised, and that customers of its main website Debenhams.com "can be confident they are unaffected by this attack".
"All affected customers have been contacted by Debenhams to inform them of the incident," the firm told Sky News.
"We are working with Ecomnova to ask the banks of those affected to block payment cards of those customers affected and issue customers with new cards."
Debenhams said the incident had been reported to the Information Commissioner's Office (ICO), the UK's independent body for upholding the Data Protection Act.
Following a cyber attack in October 2015, the ICO fined TalkTalk a record £400,000 after 15,656 individuals' bank account details and sort codes were stolen.
An ICO spokesperson said it was aware of the "potential incident" involving Debenhams Flowers and that enquiries were being made.
"Businesses and organisations are required under the Data Protection Act to keep people's personal data safe and secure," the spokesperson said.
Debenhams chief executive Sergio Bucher said: "As soon as we were informed that there had been a cyber attack, we suspended the Debenhams Flowers website and commenced a full investigation.
"We are very sorry that customers have been affected by this incident and we are doing everything we can to provide advice to affected customers and reduce their risk."
Ecomnova did not immediately respond to Sky News for comment.
Source: Sky News