TFG London, the owner of fashion chains Phase Eight and Hobbs, said it had identified a "cybersecurity incident" on Sunday and apologised to customers for any disruption caused.
The company did not provide immediate details about the nature of the incident or the extent of any operational impact, but its statement acknowledged the disruption and offered apologies to customers affected by the issue.
Separately, Swedish retail giant H&M said it was probing a reported claim that it had been the target of a cyber-attack. H&M gave no further specifics on the alleged incident as the inquiry continued.
These incidents add to a growing number of cyber security challenges faced by retailers, where online and backend systems can be vulnerable to disruption. Both companies are likely to be scrutinising their systems and communications as they investigate and respond to the reported problems.